execmem: enforce allocation size aligment to PAGE_SIZE

execmem: enforce allocation size aligment to PAGE_SIZE

Before introduction of ROX cache execmem allocation size was always
implicitly aligned to PAGE_SIZE inside vmalloc.

However, when allocation happens from the ROX cache, this is not
enforced.

Make sure that the allocation size is always consistently aligned to
PAGE_SIZE.

Mike said:

: Right now it'll make the maple trees in execmem_cache more compact.
: And it's a precaution for the case when execmem callers would want to
: change permissions on unaligned range because that would WARN_ON()
: loudly.

Peter said

: It should not have a runtime effect -- currently all this code is used
: with PAGE_SIZE multiples and everything just works.  But whilst I was
: perusing this code, I noticed that nothing actually enforced this.  If
: someone were to break this assumption things will go sideways.

Link: https://lkml.kernel.org/r/20250423144808.1619863-1-rppt@kernel.org
Fixes: 2e45474ab14f ("execmem: add support for cache of large ROX pages")
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>