Remove references to splbio in ffs_softdep.c.

Assert that the per-mountpoint softdep mutex is held in modified
functions that do not already have this assertion. No functional
change intended.

Reviewed by: kib, mckusick (previous version)
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D20741