ceph: exchange hardcoded value on NAME_MAX

Initially, ceph_fs_debugfs_init() had temporary
name buffer with hardcoded length of 80 symbols.
Then, it was hardcoded again for 100 symbols.
Finally, it makes sense to exchange hardcoded
value on properly defined constant and 255 symbols
should be enough for any name case.

Signed-off-by: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>
Reviewed-by: Patrick Donnelly <pdonnell@ibm.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>