xfs: remove xfs_flushinval_pages

It's just a simple wrapper around VFS functionality, and is actually
bugging in that it doesn't remove mappings before invalidating the
page cache. Remove it and replace it with the correct VFS
functionality.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Andrew Dahl <adahl@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>