CVE-2026-99132: 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 xfssyncsbbuf

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

setlabel: holds buf lock -> flushworkqueue(xfs-cil) CIL push worker: xfsbufitemunpin -> xfsbuflock(same buf)

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