mm/mempolicy: fix all kernel-doc warnings

Fix kernel-doc warnings in mempolicy.c:

  mempolicy.c:139: warning: No description found for return value of 'numa_map_to_online_node'
  mempolicy.c:2165: warning: Excess function parameter 'node' description in 'alloc_pages_vma'
  mempolicy.c:2973: warning: No description found for return value of 'mpol_parse_str'

Link: https://lkml.kernel.org/r/20211213233216.5477-1-rdunlap@infradead.org
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>