libpagemap: add pm_map_usage_flags

Add pm_map_usage_flags, which is the same as pm_map_usage but only
counts pages with the specified flags set.  This can be used to
only count "swapbacked" pages, which are pages that cannot be
flushed back to disk without using swap.

Change-Id: I6367555d9385502c797935849bb4221a8354e251
2 files changed