ceph: remove unnecessary IS_ERR() check in ceph_fname_to_usr()

ceph: remove unnecessary IS_ERR() check in ceph_fname_to_usr()

Before returning, function ceph_fname_to_usr() does a final IS_ERR() check
in 'dir':

	if ((dir != fname->dir) && !IS_ERR(dir)) {...}

This check is unnecessary because, if the 'dir' variable has changed to
something other than 'fname->dir' (it's initial value), that error check has
been performed already and, if there was indeed an error, it would have
been returned immediately.

Besides, this useless IS_ERR() is also confusing static analysis tools.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/r/202309282202.xZxGdvS3-lkp@intel.com/
Signed-off-by: Luis Henriques <lhenriques@suse.de>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>