dm thin metadata: make dm_thin_find_mapped_range() atomic

Refactor dm_thin_find_mapped_range() so that it takes the read lock on
the metadata's lock; rather than relying on finer grained locking that
is pushed down inside dm_thin_find_next_mapped_block() and
dm_thin_find_block().

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>