9p: Remove unnecessary IS_ERR() check

The "fid" variable can't be an error pointer so there is no need to
check.  The code is slightly cleaner if we move the increment before
the break and remove the NULL check as well.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>