mnt: Factor out unhash_mnt from detach_mnt and umount_tree

mnt: Factor out unhash_mnt from detach_mnt and umount_tree

commit 7bdb11de8ee4f4ae195e2fa19efd304e0b36c63b upstream.

Create a function unhash_mnt that contains the common code between
detach_mnt and umount_tree, and use unhash_mnt in place of the common
code.  This add a unncessary list_del_init(mnt->mnt_child) into
umount_tree but given that mnt_child is already empty this extra
line is a noop.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>