sys/vfs/hammer: Don't use HAMMER_CURSOR_GET_LEAF
HAMMER_CURSOR_GET_LEAF is not used by the current implementation
of hammer_btree_extract(), since this function will cause
cursor->leaf to point to the node element in question regardless
of the flag value. Then just don't use this flag when calling
hammer_btree_extract() when we know loading node element (leaf)
is the default behavior.
The cursor flag is already complicated enough, so simplifying
btree extract callers by either passing 0 or DATA but not LEAF
makes things a bit more clear.