xfs: create a helper to compute the blockcount of a max sized remote value

Create a helper function to compute the number of fsblocks needed to
store a maximally-sized extended attribute value.

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