rpmb: Remove some useless locking

There is no need for explicit locking when using the ida API, as stated in
the doc related to ida_alloc_range() / ida_free().

So remove rpmb_mutex.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/b1fcc6707ec2b6309d50060fa52ccc2c892afde2.1728507153.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>