MFC r294102:

MFV r294101: 6527 Possible access beyond end of string in zpool comment

Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Approved by: Gordon Ross <gwr@nexenta.com>

illumos/illumos-gate@2bd7a8d078223b122d65fea49bb8641f858b1409

This fixes erroneous double increments of the 'check' variable in a loop
in spa_prop_validate(). I ran into this in the clang380-import branch,
where clang 3.8.0 warns about it. (It is already fixed there.)