sys/vfs/hammer: Add hammer_btree_extract_leaf()

sys/vfs/hammer: Add hammer_btree_extract_leaf()

hammer_btree_extract() doesn't read data from block devices
unless flag has HAMMER_CURSOR_GET_DATA. It doesn't really matter
if the flag is HAMMER_CURSOR_GET_LEAF or not in order to just
extract the node element (without reading data) as long as the
flag doesn't have HAMMER_CURSOR_GET_DATA. Calling this function
will cause cursor->leaf to point to the node element in question
regardless of the flag value.

This commit adds hammer_btree_extract_leaf() which is just a
wrapper over hammer_btree_extract(cursor, HAMMER_CURSOR_GET_LEAF).
This hides complexity of using HAMMER_CURSOR_GET_LEAF or 0 or
anything other than HAMMER_CURSOR_GET_DATA which are all the same.