Normally when a buffer with B_BARRIER is written, the flag is cleared
by g_vfs_strategy() when creating bio.  But in some cases FFS buffer
might not reach g_vfs_strategy(), for instance when copy-on-write
reports an error like ENOSPC.  In this case buffer is returned to
dirty queue and might be written later by other means.  Among then
bdwrite() reasonably asserts that B_BARRIER is not set.

In fact, the only current use of B_BARRIER is for lazy inode block
initialization, where write of the new inode block is fenced against
cylinder group write to mark inode as used.  The situation could be
seen that we break dependency by updating cg without written out
inode.  Practically since CoW was not able to find space for a copy of
inode block, for the same reason cg group block write should fail.

Reported by:	pho
Discussed with:	chs, imp, mckusick
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D26511