NFSD: Define a proc_layoutcommit for the FlexFiles layout type

Avoid a crash if a pNFS client should happen to send a LAYOUTCOMMIT
operation on a FlexFiles layout.

Reported-by: Robert Morris <rtm@csail.mit.edu>
Closes: https://lore.kernel.org/linux-nfs/152f99b2-ba35-4dec-93a9-4690e625dccd@oracle.com/T/#t


Cc: Thomas Haynes <loghyr@hammerspace.com>
Cc: stable@vger.kernel.org
Fixes: 9b9960a0 ("nfsd: Add a super simple flex file server")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>