generic: new case to test getcwd(2)

The following kernel commit introduced a race condition that causes
getcwd(2) to return "/" instead of correct path

232d2d6 dcache: Translating dentry into pathname without taking rename_lock

Jan Stancek hit it once when building ltp and Mikulas Patocka could
hit it by running lvm2 test suite. Please refer to this thread

https://www.mail-archive.com/ltp-list@lists.sourceforge.net/msg17896.html

These commits fixed the bug
ede4ceb prepend_path() needs to reinitialize dentry/vfsmount/mnt on restarts
f650080 __dentry_path() fixes

Cc: Artem Savkov <asavkov@redhat.com>
Cc: Jan Stancek <jstancek@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

.gitignore

@@ -83,6 +83,7 @@
 /src/t_dir_offset
 /src/t_dir_offset2
 /src/t_futimens
+/src/t_getcwd
 /src/t_holes
 /src/t_immutable
 /src/t_mmap_writev