gnttab: Fix scan-build core.CallAndMessage issues

gnttab: Fix scan-build core.CallAndMessage issues

scan-build complains:

  arch/x86/grant_table.c:41:17: warning: 1st function call argument is an uninitialized value [core.CallAndMessage]
                  pte_from_gfn(gnttab_gfns[i], PF_SYM(AD, RW, P)), UVMF_INVLPG);
                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

because it can't observe that GNTTABOP_setup_table fills the variable on its
success path.

Initialising the array (which is currently one entry) isn't trivial because
the array is variadic.  Drop the nr_frames variable and use the sizeof()
expression directly, to create compile-time constant size.

A similar issue is reported against xsa-255, but this is trivial to resolve.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>