[XFS] Fix check for block zero access in xfs_write_iomap_allocate()

The check for block zero access should be done on non-realtime inodes. Fix
the logic error in xfs_write_iomap_allocate(), and simplify the logic on
all checks for block zero access in xfs_iomap.c

SGI-PV: 980888
SGI-Modid: xfs-linux-melb:xfs-kern:30998a

Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>