x86: xen: remove unnecessary variable in xen_foreach_remap_area()

Remove unnecessary variable mfn in function xen_foreach_remap_area() and,
refactor the code.

Variable mfn at line 518:mfn = xen_remap_buf.mfns[i];
is only being used to store a value to be passed as
an argument to the xen_update_mem_tables() function.
This value can be passed directly, which makes variable
mfn unnecessary. Also, value assigned to variable mfn
at line 534:mfn = xen_remap_mfn; is never used.

Addresses-Coverity-ID: 1260110
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>