[XFS] Move attr log alloc size calculator to another function.

We will need that to be able to calculate the size of log we need for a
specific attr (for Create+EA). The local flag is needed so that we can
fail if we run into ENOSPC when trying to alloc blocks.

SGI-PV: 981498

SGI-Modid: xfs-linux-melb:xfs-kern:31727a

Signed-off-by: Niv Sardi <xaiki@sgi.com>
Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Tim Shimmin <tes@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>