We defind alloc_metapage, but instead of using it, the potential callers
are calling mempool_alloc directly.  Since we have and use the function,
free_metapage, we should be consistent and use alloc_metapage.

This patch also changes the argument to alloc_metapage so that the
caller passes gfp_mask directly, rather than a less descriptive flag.
Signed-off-by: Dave Kleikamp <shaggy@austin.ibm.com>