combine __put_mountpoint() with unhash_mnt()

combine __put_mountpoint() with unhash_mnt()

A call of unhash_mnt() is immediately followed by passing its return
value to __put_mountpoint(); the shrink list given to __put_mountpoint()
will be ex_mountpoints when called from umount_mnt() and list when called
from mntput_no_expire().

Replace with __umount_mnt(mount, shrink_list), moving the call of
__put_mountpoint() into it (and returning nothing), adjust the
callers.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>