xfs: set buf types when converting extent formats

Conversion from local to extent format does not set the buffer type
correctly on the new extent buffer when a symlink data is moved out
of line.

Fix the symlink code and leave a comment in the generic bmap code
reminding us that the format-specific data copy needs to set the
destination buffer type appropriately.

cc: <stable@vger.kernel.org> # 3.10 to current
Tested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>