xfs: don't hold buffer locks across sync transaction commit in xfs_sync_sb_buf

In the Linux kernel, the following vulnerability has been resolved:

xfs: don't hold buffer locks across sync transaction commit in xfs_sync_sb_buf

xfs_sync_sb_buf() holds sb/rtsb buffer locks across a synchronous
xfs_trans_commit(), which flushes the CIL push workqueue internally.
If shutdown occurs during the CIL push, xfs_buf_item_unpin() needs to
lock these buffers to fail them, causing a deadlock:

setlabel: holds buf lock -> flush_workqueue(xfs-cil)
CIL push worker: xfs_buf_item_unpin -> xfs_buf_lock(same buf)

Remove the xfs_trans_bhold() calls so that commit releases the buffer
locks normally. After the sync commit, re-acquire the buffers via
mp->m_sb_bp / mp->m_rtsb_bp for the on-disk writeback.