x86/mm: Fix IS_ALIGNED() check in IS_LnE_ALIGNED()

The current CI failures turn out to be a latent bug triggered by a narrow set
of properties of the initrd and the host memory map, which CI encountered by
chance.

One step during boot involves constructing directmap mappings for modules.
With some probing at the point of creation, it is observed that there's a 4k
mapping missing towards the end of the initrd.

  (XEN) === Mapped Mod1 [0000000394001000, 00000003be1ff6dc] to Directmap
  (XEN) Probing paddr 394001000, va ffff830394001000
  (XEN) Probing paddr 3be1ff6db, va ffff8303be1ff6db
  (XEN) Probing paddr 3bdffffff, va ffff8303bdffffff
  (XEN) Probing paddr 3be001000, va ffff8303be001000
  (XEN) Probing paddr 3be000000, va ffff8303be000000
  (XEN) Early fatal page fault at e008:ffff82d04032014c (cr2=ffff8303be000000, ec=0000)

The conditions for this bug appear to be map_pages_to_xen() call with a start
address of exactly 4k beyond a 2M boundary, some number of full 2M pages, then
a tail needing 4k pages.

Anyway, the condition for spotting superpage boundaries in map_pages_to_xen()
is wrong.  The IS_ALIGNED() macro expects a power of two for the alignment
argument, and subtracts 1 itself.

Fixing this causes the failing case to now boot.

Fixes: 97fb6fcf26 ("x86/mm: introduce helpers to detect super page alignment")
Debugged-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Tested-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/mm.c

@@ -5505,7 +5505,7 @@ int map_pages_to_xen(
                                                                 \
     ASSERT(!mfn_eq(m_, INVALID_MFN));                           \
     IS_ALIGNED(PFN_DOWN(v) | mfn_x(m_),                         \
-               (1UL << (PAGETABLE_ORDER * ((n) - 1))) - 1);     \
+               1UL << (PAGETABLE_ORDER * ((n) - 1)));           \
 })
 #define IS_L2E_ALIGNED(v, m) IS_LnE_ALIGNED(v, m, 2)
 #define IS_L3E_ALIGNED(v, m) IS_LnE_ALIGNED(v, m, 3)