ubd: Remove unused mutex 'ubd_mutex'

Commit fb5d1d38 ("ubd: open the backing files in ubd_add")

removed the last use of ubd_mutex.
Remove it.

Build and kernel startup test only.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://patch.msgid.link/20240505001508.255096-1-linux@treblig.org


Signed-off-by: Johannes Berg <johannes.berg@intel.com>