mnt: If fs_fully_visible fails call put_filesystem.

mnt: If fs_fully_visible fails call put_filesystem.

Orabug: 25256941

[ Upstream commit 97c1df3e54e811aed484a036a798b4b25d002ecf ]

Add this trivial missing error handling.

Cc: stable@vger.kernel.org
Fixes: 1b852bceb0d1 ("mnt: Refactor the logic for mounting sysfs and proc in a user namespace")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
(cherry picked from commit fced2a819164c305544bcc8d9a45beea15374528)
Signed-off-by: Dhaval Giani <dhaval.giani@oracle.com>