UFS: clear fs_fmod once more, in the buffer data copy.
This is needed for in-kernel copy of the code, where allocation might
happen after fs_fmod is cleared in ffs_sbput() but before the write.
Reported by: markj
Reviewed by: chs, markj
PR: 263765
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D35149