cif: fix dead code

This issue was found by Coverity (CID 1202536)

This proposes a fix for a statement that creates dead code.
The "rc < 0" statement is within code that is run
with "rc > 0".

It seems like "err < 0" was meant to be used here.
This way, the error code is returned by the function.
Signed-off-by: Michael Opdenacker <michael.opdenacker@free-electrons.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Steve French <smfrench@gmail.com>

fs/cifs/file.c

@@ -2599,7 +2599,7 @@ cifs_writev(struct kiocb *iocb, const struct iovec *iov,
 			ssize_t err;
 
 			err = generic_write_sync(file, iocb->ki_pos - rc, rc);
-			if (rc < 0)
+			if (err < 0)
 				rc = err;
 		}
 	} else {