xfs: improve the code that checks recovered bmap intent items

The code that validates recovered bmap intent items is kind of a mess --
it doesn't use the standard xfs type validators, and it doesn't check
for things that it should.  Fix the validator function to use the
standard validation helpers and look for more types of obvious errors.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>