GFS2: pass leaf_bh into leaf_dealloc

Function foreach_leaf used to look up the leaf block address and get
a buffer_head.  Then it would call leaf_dealloc which did the same
lookup.  This patch combines the two operations by making foreach_leaf
pass the leaf bh to leaf_dealloc.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>