xfs: only writeback and truncate pages for the freed range

xfs_free_file_space() only affects the range of the file for which space
is being freed. It currently writes and truncates the page cache from
the start offset of the free to EOF.

Modify xfs_free_file_space() to write back and truncate page cache of
just the range being freed.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>