xfs_db: fix broken logic in error path

xfs_db: fix broken logic in error path

smatch complains proceeding into the if body if leaf is a null pointer:

check.c:3614 process_leaf_node_dir_v2_int() warn: variable dereferenced before check 'leaf' (see line 3518)

However, the logic here is misleading and broken -- what we're trying to
do is switch between the v4 and v5 variants of the directory check.
We're using @leaf3 being a null pointer (or not) to determine v4 vs. v5,
so the "!" part of the comparison is correct, but the variable used
(leaf) is not.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>