nfscl: Fix handling of a copyout() error reply

If vfs.nfs.nfs_directio_enable is set non-zero (the default is
zero) and a file on an NFS mount is read after being opened
with O_DIRECT | O_ RDONLY, a call to nfsm_mbufuio() calls
copyout() without checking for an error return.
If copyout() returns EFAULT, this would not work correctly.

Only the call path
VOP_READ()->ncl_readrpc()->nfsrpc_read()->nfsrpc_readrpc()
will do this and the error return for EFAULT will
be returned back to VOP_READ().

This patch adds the error check to nfsm_mbufuio().

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D43160