nfs: nfs_file_write() should check for writeback errors

The NFS_CONTEXT_ERROR_WRITE flag (as well as the check of said flag) was
removed by commit 6fbda89b.  The absence of an error check allows
writes to be continually queued up for a server that may no longer be
able to handle them.  Fix it by adding an error check using the generic
error reporting functions.

Fixes: 6fbda89b

 ("NFS: Replace custom error reporting mechanism with generic one")
Signed-off-by: Scott Mayhew <smayhew@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>