Call softdep_prealloc() before taking ffs_lock_ea(), if unlock is committing

softdep_prealloc() must be called to ensure enough journal space is
available, before ffs_extwrite(). Also it must be done before taking
ffs_lock_ea(), because it calls ffs_syncvnode(), potentially dropping
the vnode lock.

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