nfsd: define xattr functions to call into their vfs counterparts

This adds the filehandle based functions for the xattr operations
that call in to the vfs layer to do the actual work.

Signed-off-by: Frank van der Linden <fllinden@amazon.com>
[ cel: address checkpatch.pl complaint ]
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>