mm/vmalloc: add 'align' parameter explanation for pvm_determine_end_from_reverse

Kernel-doc markup has a issue on pvm_determine_end_from_reverse:

  mm/vmalloc.c:3145: warning: Function parameter or member 'align' not described in 'pvm_determine_end_from_reverse'

Add a explanation for it to remove the warning.

Link: https://lkml.kernel.org/r/1605605088-30668-3-git-send-email-alex.shi@linux.alibaba.comSigned-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmalloc.c

@@ -3151,6 +3151,7 @@ pvm_find_va_enclose_addr(unsigned long addr)
  * @va:
  *   in - the VA we start the search(reverse order);
  *   out - the VA with the highest aligned end address.
+ * @align: alignment for required highest address
  *
  * Returns: determined end address within vmap_area
  */