x86/mm: Fix possible increment of uninitialised variable

x86/mm: Fix possible increment of uninitialised variable

Discovered by Coverity, CID 1056101

When taking the continue branch on the first iteration of the loop,
gfn would indeed be uninitialised when incremented.  However, as gfn
is unconditionally constructed from i{1..4} before use in the loop
body, having it incremented in the loop header is useless.

Therefore, simply remove it.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Tim Deegan <tim@xen.org>