ocfs2: fix subdirectory registration with register_sysctl()

The kernel test robot reports that commit c42ff46f ("ocfs2: simplify
subdirectory registration with register_sysctl()") is broken, and
results in kernel warning messages like

  sysctl table check failed: fs/ocfs2/nm Not a file
  sysctl table check failed: fs/ocfs2/nm No proc_handler
  sysctl table check failed: fs/ocfs2/nm bogus .mode 0555

and in fact this was already reported back in linux-next, but nobody
seems to have reacted to that report.  Possibly that original report
only ever made it to the lkp list.

The problem seems to be that the simplification didn't actually go far
enough, and should have converted the whole directory path to the final
sysctl file, rather than just the two first components.

So take that last step.

Fixes: c42ff46f

 ("ocfs2: simplify subdirectory registration with register_sysctl()")
Reported-by: kernel test robot <oliver.sang@intel.com>
Link: https://lore.kernel.org/all/20220128065310.GF8421@xsang-OptiPlex-9020/
Link: https://lists.01.org/hyperkitty/list/lkp@lists.01.org/thread/KQ2F6TPJWMDVEXJM4WTUC4DU3EH3YJVT/


Tested-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>