xfs_db: improve number extraction in getbitval

xfs_db: improve number extraction in getbitval

For some reason, getbitval insists upon collecting a u64 from a pointer
bit by bit if it's not aligned to a 16-byte boundary.  If not, then it
resorts to scraping bits individually.  I don't know of any platform
where we require 16-byte alignment for a 8-byte access, or why we'd care
now that we have things like get_unaligned_beXX.

Rework this function to detect either naturally aligned accesses and use
the regular beXX_to_cpu functions; or byte-aligned accesses and use the
get_unaligned_beXX functions.  Only fall back to the bit scraping
algorithm for the really weird cases.

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