NFSD: fix LISTXATTRS returning more bytes than maxcount

The maxcount is the maximum number of bytes for the LISTXATTRS4resok
result. This includes the cookie and the count for the name array,
thus subtract 12 bytes from the maxcount: 8 (cookie) + 4 (array count)
when filling up the name array.

Fixes: 23e50fe3

 ("nfsd: implement the xattr functions and en/decode logic")
Signed-off-by: Jorge Mora <mora@netapp.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>