ufs vnops: brace softdep_prelink() with DOINGSUJ instead of DOINGSOFTDEP

because softdep_prelink() is reverted to NOP for non-J case. There is no
need to do anything before ufs_direnter() in SU/non-J case, everything
required to sync the directory is done in VOP_VPUT_PAIR().

Suggested by: mckusick
Reviewed by: chs, mckusick
Tested by: pho
MFC after: 2 week
Sponsored by: The FreeBSD Foundation