LU-4416 osd-ldiskfs: remove unused 'optional_mutex' argument

osd_ldiskfs_map_inode_pages doesn't actually use the
optional_mutex argument. Removing it now makes it easier to
integrate the ext4_map_blocks changes.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: James Simmons <uja.ornl@gmail.com>
Change-Id: Ic87114b8d7adadf386a0794c2e6ece79c83601b4
Reviewed-on: http://review.whamcloud.com/10162
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>