mm/mseal: simplify and rename VMA gap check

mm/mseal: simplify and rename VMA gap check

The check_mm_seal() function is doing something general - checking whether
a range contains only VMAs (or rather that it does NOT contain any
unmapped regions).

So rename this function to range_contains_unmapped().

Additionally simplify the logic, we are simply checking whether the last
vma->vm_end has either a VMA starting after it or ends before the end
parameter.

This check is rather dubious, so it is sensible to keep it local to
mm/mseal.c as at a later stage it may be removed, and we don't want any
other mm code to perform such a check.

No functional change intended.

[lorenzo.stoakes@oracle.com: add comment explaining why we disallow gaps on mseal()]
Link: https://lkml.kernel.org/r/d85b3d55-09dc-43ba-8204-b48267a96751@lucifer.local
Link: https://lkml.kernel.org/r/dd50984eff1e242b5f7f0f070a3360ef760e06b8.1753431105.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Jeff Xu <jeffxu@chromium.org>
Reviewed-by: Pedro Falcato <pfalcato@suse.de>
Cc: Jann Horn <jannh@google.com>
Cc: Kees Cook <kees@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>