mm: fix filemap.c kernel doc warning

Add missing kernel-doc notation for function parameter 'gfp_mask' to fix
kernel-doc warning.

  mm/filemap.c:1898: warning: No description found for parameter 'gfp_mask'
Signed-off-by: NRandy Dunlap <rdunlap@infradead.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/filemap.c

@@ -1890,6 +1890,7 @@ EXPORT_SYMBOL(generic_file_read_iter);
  * page_cache_read - adds requested page to the page cache if not already there
  * @file:	file to read
  * @offset:	page index
+ * @gfp_mask:	memory allocation flags
  *
  * This adds the requested page to the page cache if it isn't already there,
  * and schedules an I/O to read in its contents from disk.