[XFS] fix ASSERT and ASSERT_ALWAYS

- remove the != 0 inside the unlikely in ASSERT_ALWAYS because sparse now
  complains about comparisons between pointers and 0
- add a standalone ASSERT implementation because defining it to
  ASSERT_ALWAYS means the string is expanded before the token passing
  stringification. This way we get the actual content of the
  assertion in the assfail message and don't overflow sparse's
  stringification buffer leading to sparse error messages.

SGI-PV: 968555
SGI-Modid: xfs-linux-melb:xfs-kern:29310a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>