[XFS] remove dead SYNC_BDFLUSH case in xfs_sync_inodes

A large part of xfs_sync_inodes is conditional on the SYNC_BDFLUSH which
is never passed to it. This patch removes it and adds an assert that
triggers in case some new code tries to pass SYNC_BDFLUSH to it.

SGI-PV: 970242
SGI-Modid: xfs-linux-melb:xfs-kern:29630a

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