generic: test record locks across execve in multithread process

POSIX requires that record locks are preserved across an execve(2).
But currently the locks are released if process is multithreaded at
the time that execve is called.

As Jeff Layton wrote in his patch:
"
In that case, we'll end up unsharing the files_struct but the locks
will still have their fl_owner set to the address of the old one.
Eventually, when the other threads die and the last reference to the
old files_struct is put, any POSIX locks get torn down since it
looks like a close occurred on them.

The result is that all of your open files will be intact with none
of the locks you held before execve.
"

Add a new regression test for this particular case.

[Eryu: rewrite commit log and test description]

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

.gitignore

@@ -121,6 +121,7 @@
 /src/t_getcwd
 /src/t_holes
 /src/t_immutable
+/src/t_locks_execve
 /src/t_mmap_cow_race
 /src/t_mmap_dio
 /src/t_mmap_fallocate