src/open_by_handle: flexible usage options

More usage options for testing open_by_handle, which are needed
for testing stable handles across copy up in overlayfs.

usage: open_by_handle [-c|-l|-u|-d] <test_dir> [num_files]

Examples:

1. Create test set of N files and try to get their NFS handles:

   open_by_handle -c <test_dir> [N]

   This is used by new helper _require_exportfs() to check
   if filesystem supports exportfs

2. Get file handles for existing test set, drop caches and try to
   open all files by handle:

   open_by_handle <test_dir> [N]

3. Get file handles for existing test set, unlink all test files,
   drop caches, try to open all files by handle and expect ESTALE:

   open_by_handle -d <test_dir> [N]

4. Get file handles for existing test set, hardlink all test files,
   then unlink the original files, drop caches and try to open all
   files by handle (should work):

   open_by_handle -l <test_dir> [N]
   open_by_handle -u <test_dir> [N]

   This test is done with 2 invocations of the program, first to
   hardlink (-l) and then to unlink the originals (-u), because
   we would like to be able to perform the hardlinks on overlay
   lower layer and unlink on upper layer.

   NOTE that open_by_handle -u doesn't check if the files are
   hardlinked, it just assumes that they are.  If they are not
   then the test will fail, because file handles would be stale.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>

doc/auxiliary-programs.txt

@@ -15,9 +15,10 @@ note the dependency with:
 
 Contents:
 
- - af_unix	-- Create an AF_UNIX socket
- - stat_test	-- statx syscall exercise
- - xfs_io	-- General I/O operation exercise
+ - af_unix		-- Create an AF_UNIX socket
+ - open_by_handle	-- open_by_handle_at syscall exercise
+ - stat_test		-- statx syscall exercise
+ - xfs_io		-- General I/O operation exercise
 
 
 ==================
@@ -28,6 +29,15 @@ af_unix
 
 	The af_unix program creates an AF_UNIX socket at the given location.
 
+open_by_handle
+
+	The open_by_handle program exercises the open_by_handle_at() system
+	call.  It can check if file handles are valid or stale after certain
+	filesystem operations.
+
+	See also:
+		_require_exportfs
+
 stat_test
 
 	The stat_test program is primarily designed to exercise the statx()

doc/requirement-checking.txt

@@ -15,6 +15,7 @@ they have.  This is done with _require_<xxx> macros, which may take parameters.
  (2) Filesystem capability requirements.
 
 	_require_chattr <letters>
+	_require_exportfs
 
  (3) System call requirements.
 
@@ -86,6 +87,12 @@ _require_chattr <letters>
      tests to see if setting the append-only and immutable attributes on a file
      (chattr +a +i) is supported.
 
+_require_exportfs
+
+     The test requires that the $TEST_DEV filesystem supports NFS export.
+     The test also requires the use of the open_by_handle_at() system call and
+     will be skipped if it isn't available in the kernel.
+
 
 ========================
 SYSTEM CALL REQUIREMENTS