mm: fix page_alloc.c kernel-doc warnings

Fix kernel-doc warnings and function name in mm/page_alloc.c:

  Warning(..//mm/page_alloc.c:6074): No description found for parameter 'pfn'
  Warning(..//mm/page_alloc.c:6074): No description found for parameter 'mask'
  Warning(..//mm/page_alloc.c:6074): Excess function parameter 'start_bitidx' description in 'get_pfnblock_flags_mask'
  Warning(..//mm/page_alloc.c:6102): No description found for parameter 'pfn'
  Warning(..//mm/page_alloc.c:6102): No description found for parameter 'mask'
  Warning(..//mm/page_alloc.c:6102): Excess function parameter 'start_bitidx' description in 'set_pfnblock_flags_mask'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Mel Gorman <mgorman@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>