NFS: Fix error reporting in nfs_file_write()

When doing O_DSYNC writes, the actual write errors are reported through
generic_write_sync(), so we must test the result.

Reported-by: J. R. Okajima <hooanon05g@gmail.com>
Fixes: 18290650 ("NFS: Move buffered I/O locking into nfs_file_write()")
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>