ffs: do not call softdep_prealloc() from UFS_BALLOC()

Do it in ffs_write(), where we can gracefuly handle relock and its
consequences. In particular, recheck the v_data to see if the vnode
reclamation ended, and return EBADF when we cannot proceed with the
write.

Reviewed by: mckusick
Reported by: pho
MFC after: 1 week
Sponsored by: The FreeBSD Foundation