NFSD: Use unsigned length argument for decode_filename

Clean up: file name lengths are unsigned on the wire, negative lengths
are not meaningful natively either.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Acked-By: NeilBrown <neilb@suse.de>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>