xfs: move page freeing into _xfs_buf_free_pages()

Rather than open coding it just before we call
_xfs_buf_free_pages(). Also, rename the function to
xfs_buf_free_pages() as the leading underscore has no useful
meaning.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>