generic: add OFD lock tests

Test OFD locks. Use fcntl F_OFD_SETLK/F_OFD_GETLK, to verify we are
being given correct advices through getlk by kernel.

The basic idea is one setlk routine setting locks via fcntl *_SETLK,
followed by operations like clone, dup then close fd; another
routine getlk getting locks via fcntl *_GETLK.

Firstly in setlk routine process P0, place a lock L0 on an opened
testfile, then do clone or dup and close relative fd.

In getlk process P2, do fcntl *_GETLK with lock L1 after get
notified by setlk routine.

In the end, getlk routine check the returned struct flock.l_type to
see if the lock mechanism works fine.

Test combainations of:
- shared or exclusive lock
- these locks are conflicting or not
- one OFD lock and one POSIX lock
- that open testfile RDONLY or RDWR
- clone with CLONE_FILES or not
- dup and close newfd

[eguan: made some minor non-functional changes]

Signed-off-by: Xiong Zhou <xzhou@redhat.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>

.gitignore

@@ -128,6 +128,7 @@
 /src/t_mmap_write_ro
 /src/t_mmap_writev
 /src/t_mtab
+/src/t_ofd_locks
 /src/t_readdir_1
 /src/t_readdir_2
 /src/t_rename_overwrite