Do not leak transient ENOLCK error from flush_newblk_dep() loop.

The buffer lock is retried on failed LK_SLEEPFAIL attempt, and error
from the failed attempt is irrelevant.  But since there is path after
retry which does not clear error, it is possible to return spurious
error from the function.

The issue resulted in a spurious failure of softdep_sync_buf(),
causing further spurious failure of ffs_sync().

In collaboration with:	pho
Reviewed by:	mckusick
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks

sys/ufs/ffs/ffs_softdep.c

@@ -12954,6 +12954,7 @@ flush_newblk_dep(vp, mp, lbn)
 			    LK_INTERLOCK, BO_LOCKPTR(bo));
 			if (error == ENOLCK) {
 				ACQUIRE_LOCK(ump);
+				error = 0;
 				continue; /* Slept, retry */
 			}
 			if (error != 0)