do_new_mount_fc(): use __free() to deal with dropping mnt on failure

do_new_mount_fc(): use __free() to deal with dropping mnt on failure

do_add_mount() consumes vfsmount on success; just follow it with
conditional retain_and_null_ptr() on success and we can switch
to __free() for mnt and be done with that - unlock_mount() is
in the very end.

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