x86/soft-reset: Drop gfn reference after calling get_gfn_query()

x86/soft-reset: Drop gfn reference after calling get_gfn_query()

get_gfn_query() internally takes the p2m lock, and this error path leaves it
locked.

This wasn't included in XSA-277 because the error path can only be triggered
by a carefully timed phymap operation concurrent with the domain being paused
and the toolstack issuing DOMCTL_soft_reset.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
master commit: e7969e917cef276318f722a607985a2e896aeb94
master date: 2018-11-22 17:58:46 +0000