open_by_handle: test content of open file handle

open_by_handle: test content of open file handle

usage: open_by_handle -rwa <test_dir> [N]

Get file handles for existing test set, write data to files,
drop caches, open all files by handle, read data and verify old data,
write new data to files.

This is needed for testing that overlay decoded file handles are not
pointing the lower inodes after new data is already written to upper
inodes after copy up.

open_by_handle -a is needed for testing copy up of disconnected overlay
decoded file handles (to index dir).

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