ceph: fix variable dereferenced before check in ceph_umount_begin()

ceph: fix variable dereferenced before check in ceph_umount_begin()

smatch warnings:
fs/ceph/super.c:1042 ceph_umount_begin() warn: variable dereferenced before check 'fsc' (see line 1041)

vim +/fsc +1042 fs/ceph/super.c

void ceph_umount_begin(struct super_block *sb)
{
	struct ceph_fs_client *fsc = ceph_sb_to_fs_client(sb);

	doutc(fsc->client, "starting forced umount\n");
              ^^^^^^^^^^^
Dereferenced

	if (!fsc)
            ^^^^
Checked too late.

		return;
	fsc->mount_state = CEPH_MOUNT_SHUTDOWN;
	__ceph_umount_begin(fsc);
}

The VFS guarantees that the superblock is still
alive when it calls into ceph via ->umount_begin().
Finally, we don't need to check the fsc and
it should be valid. This patch simply removes
the fsc check.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/r/202503280852.YDB3pxUY-lkp@intel.com/
Signed-off-by: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>
Reviewed by: Alex Markuze <amarkuze@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>