Fix a call to copyout() so that it checks for an error return

[rmacklem]

Summary

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().

Test Plan

Hacked the code to artificially make nfsm_mbufuio() return
EFAULT instead of doing the copyout() call intermittently
and then did a O_DIRECT | O_RDONLY read of a fair sized
file with vfs.nfs.nfs_directio_enable set to 1.

This worked correctly, so any return of EFAULT by
copyout() should behave correctly as well.

[markj]

Thanks!