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>