LU-9611 lod: allow -1 for default stripe count/offset

LU-9611 lod: allow -1 for default stripe count/offset

Since LU-7344 patch http://review.whamcloud.com/16930 was landed,
lod_stripeoffset_seq_write() and lod_stripecount_seq_write() have
incorrectly checked that lod.*.stripecount and lod.*.stripeoffset
are not negative.  In fact they can both be "-1" to indicate that
the filesystem-wide default value should be used. These parameters
can also be set internally if using "lfs setstripe -c -1 $MOUNT"
or "lfs setstripe -i -1 $MOUNT" to set the system wide default,
generating console errors on the MDS from class_process_proc_param():

  lov.: error writing proc entry 'stripecount': rc = -34
  lov.: error writing proc entry 'stripeoffset': rc = -34

Add comment blocks to lod_fix_desc_stripe*() to indicate their use.
Fix these functions to allow "-1" as a valid value.
Fix the error message to properly indicate which device is being
modified, in case multiple filesystems are mounted.

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: I295d2591d535b039634689524a29725e96ce2c01
Reviewed-on: https://review.whamcloud.com/27473
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>