x86/pv: Fix undefined behaviour in check_descriptor()

UBSAN reports:

  (XEN) ================================================================================
  (XEN) UBSAN: Undefined behaviour in x86_64/mm.c:1108:31
  (XEN) left shift of 255 by 24 places cannot be represented in type 'int'
  (XEN) ----[ Xen-4.13-unstable  x86_64  debug=y   Tainted:    H ]----
  (XEN) CPU:    60
  (XEN) RIP:    e008:[<ffff82d0802a54ce>] ubsan.c#ubsan_epilogue+0xa/0xc2
  <snip>
  (XEN) Xen call trace:
  (XEN)    [<ffff82d0802a54ce>] ubsan.c#ubsan_epilogue+0xa/0xc2
  (XEN)    [<ffff82d0802a6009>] __ubsan_handle_shift_out_of_bounds+0x15d/0x16c
  (XEN)    [<ffff82d08033abd7>] check_descriptor+0x191/0x3dd
  (XEN)    [<ffff82d0804ef920>] do_update_descriptor+0x7f/0x2b6
  (XEN)    [<ffff82d0804efb75>] compat_update_descriptor+0x1e/0x20
  (XEN)    [<ffff82d0804fa1cc>] pv_hypercall+0x87f/0xa6f
  (XEN)    [<ffff82d080501acb>] do_entry_int82+0x53/0x58
  (XEN)    [<ffff82d08050702b>] entry_int82+0xbb/0xc0
  (XEN)
  (XEN) ================================================================================

As this is a constant, express it in longhand for correctness, and consistency
with the surrounding code.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/x86_64/mm.c

@@ -1105,7 +1105,7 @@ int check_descriptor(const struct domain *dom, seg_desc_t *d)
              * 0xf6800000. Extend these to allow access to the larger read-only
              * M2P table available in 32on64 mode.
              */
-            base = (b & (0xff << 24)) | ((b & 0xff) << 16) | (a >> 16);
+            base = (b & 0xff000000) | ((b & 0xff) << 16) | (a >> 16);
 
             limit = (b & 0xf0000) | (a & 0xffff);
             limit++; /* We add one because limit is inclusive. */