xfs: refactor extended attribute buffer pointer functions

Replace the open-coded attribute buffer pointer calculations with helper
functions to make it more obvious what we're doing with our freeform
memory allocation w.r.t. either storing xattr values or computing btree
block free space.

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