mm/mseal: rework mseal apply logic

mm/mseal: rework mseal apply logic

The logic can be simplified - firstly by renaming the inconsistently named
apply_mm_seal() to mseal_apply().

We then wrap mseal_fixup() into the main loop as the logic is simple
enough to not require it, equally it isn't a hugely pleasant pattern in
mprotect() etc.  so it's not something we want to perpetuate.

We eliminate the need for invoking vma_iter_end() on each loop by directly
determining if the VMA was merged - the only thing we need concern
ourselves with is whether the start/end of the (gapless) range are offset
into VMAs.

This refactoring also avoids the rather horrid 'pass pointer to prev
around' pattern used in mprotect() et al.

No functional change intended.

Link: https://lkml.kernel.org/r/ddfa4376ce29f19a589d7dc8c92cb7d4f7605a4c.1753431105.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Pedro Falcato <pfalcato@suse.de>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Jeff Xu <jeffxu@chromium.org>
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>