tools/libxc: linux: Don't use getpagesize() when unmapping the grants

tools/libxc: linux: Don't use getpagesize() when unmapping the grants

The grants are based on the Xen granularity (i.e 4KB). While the function
to map grants for Linux (linux_gnttab_grant_map) is using the correct
size (XC_PAGE_SIZE), the unmap one (linux_gnttab_munmap) is using
getpagesize().

On domain using a page granularity different than Xen (this is the case
for AARCH64 guest using 64KB page), the unmap will be called with the
wrong size.

Signed-off-by: Julien Grall <julien.grall@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>