x86/xen: use vmap() to map grant table pages in PVH guests

Commit b7dd0e35 (x86/xen: safely map and unmap grant frames when
in atomic context) causes PVH guests to crash in
arch_gnttab_map_shared() when they attempted to map the pages for the
grant table.

This use of a PV-specific function during the PVH grant table setup is
non-obvious and not needed.  The standard vmap() function does the
right thing.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reported-by: Mukesh Rathor <mukesh.rathor@oracle.com>
Tested-by: Mukesh Rathor <mukesh.rathor@oracle.com>
Cc: stable@vger.kernel.org