checkpatch: Eliminate false positive in case of comma-space-square bracket

Previously, an error was printed in cases such as:
{ [1] = 5, [2] = 6 }
The space passed OK after a curly brace, but not after a comma.
Now, a space before a square bracket is allowed, if a comma comes before
it.

Signed-off-by: Leonid Bloch <leonid@daynix.com>
Message-Id: <1446112118-12376-2-git-send-email-leonid@daynix.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
