xfs: dont cast to char * for XFS_DFORK_*PTR macros

Code in the next patch will assign the return value of XFS_DFORK_*PTR
macros to a struct pointer.  gcc complains about casting char* strings
to struct pointers, so let's fix the macro's cast to void* to shut up
the warnings.

While we're at it, fix one of the scrub tests that uses PTR to use BOFF
instead for a simpler integer comparison, since other linters whine
about char* and void* comparisons.

Can't satisfy all these dman bots.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>