GFS2: remove *leaf_call_t and simplify leaf_dealloc

Since foreach_leaf is only called with leaf_dealloc as its only possible
call function, we can simplify the code by making it call leaf_dealloc
directly.  This simplifies the code and eliminates the need for
leaf_call_t, the generic call method.  This is a first small step in
simplifying the directory leaf deallocation code.

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