ffs_use_bwrite: make the superblock snapshot more consistent

[kib]

Summary

Copy in-memory struct fs to the superblock buffer under the UFS mutex.

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
PR:     263765