mm/vmalloc: add code comment for find_vmap_area_exceed_addr()

mm/vmalloc: add code comment for find_vmap_area_exceed_addr()

Its behaviour is like find_vma() which finds an area above the specified
address, add comment to make it easier to understand.

And also fix two places of grammer mistake/typo.

Link: https://lkml.kernel.org/r/20220607105958.382076-5-bhe@redhat.com
Signed-off-by: Baoquan He <bhe@redhat.com>
Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Cc: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>