xfs: fix for hang during synchronous buffer write error

If removed storage while synchronous buffer write underway,
"xfslogd" hangs.

Detailed log http://oss.sgi.com/archives/xfs/2011-07/msg00740.html

Related work bfc60177
"xfs: fix error handling for synchronous writes"

Given that xfs_bwrite actually does the shutdown already after
waiting for the b_iodone completion and given that we actually
found that calling xfs_force_shutdown from inside
xfs_buf_iodone_callbacks was a major contributor the problem
it better to drop this call.
Signed-off-by: Ajeet Yadav <ajeet.yadav.77@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alex Elder <aelder@sgi.com>