xfs: validity check the directory block leaf entry count

The directory block format verifier fails to check that the leaf
entry count is in a valid range, and so if it is corrupted then it
can lead to derefencing a pointer outside the block buffer. While we
can't exactly validate the count without first walking the directory
block, we can ensure the count lands in the valid area within the
directory block and hence avoid out-of-block references.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>