finish_automount(): take the lock_mount() analogue into a helper

finish_automount(): take the lock_mount() analogue into a helper

finish_automount() can't use lock_mount() - it treats finding something
already mounted as "quitely drop our mount and return 0", not as
"mount on top of whatever mounted there".  It's been open-coded;
let's take it into a helper similar to lock_mount().  "something's
already mounted" => -EBUSY, finish_automount() needs to distinguish
it from the normal case and it can't happen in other failure cases.

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