[XFS] Ensure xfs_bawrite() errors are checked.

xfs_bawrite() can return immediate error status on async writes. Unlike
xfsbdstrat() we don't ever check the error on the buffer after the call,
so we currently do not catch errors at all here. Ensure we catch and
propagate or warn to the syslog about up-front async write errors.

SGI-PV: 980084
SGI-Modid: xfs-linux-melb:xfs-kern:30824a

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