ffs_reallocblks: change the guard for softdep_prealloc() call to DOINGSUJ()

instead of DOINGSOFTDEP(). The softdep_prealloc() function does nothing
in SU case.

Note that the call should be safe with regard to the vnode relock,
because it is called with MNT_NOWAIT, which does not descend into fsync.

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