xen/mm: Drop ARM put_gfn() stub

xen/mm: Drop ARM put_gfn() stub

On x86, get_gfn_*() and put_gfn() are reference counting pairs.  All the
get_gfn_*() functions are called from within CONFIG_X86 sections, but
put_gfn() is stubbed out on ARM.

As a result, the common code reads as if ARM is dropping references it never
acquired.

Put all put_gfn() calls in common code inside CONFIG_X86 to make the code
properly balanced, and drop the ARM stub.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>