TAKE - fix handling of bogus log, rt mount opts

[Eric Sandeen <sandeen@sgi.com>]

doing something like: (note the misspelled device)

$ mount -o rtdev=/dev/sad3 /dev/sda2 /mnt/sda2/

will blow up; when we go look for the device we get a negative
dentry from a previous failed lookup, and just carry on, which
will lead to a null pointer dereference... boom.

Make sure we return an error in this case, before we blow up!

Date:  Fri Feb 15 14:11:02 PST 2002
Workarea:  stout.americas.sgi.com:/localhome/eric/2.4.x-xfs/workarea-reallyclean

The following file(s) were checked into:
  bonnie.engr.sgi.com:/isms/slinx/2.4.x-xfs


Modid:  2.4.x-xfs:slinx:111941a
linux/fs/xfs/linux/xfs_super.c - 1.156
	- Watch out for negative dentries when looking up log, rtdev devices from
	  mount options.