ext4: use swap macro in mext_page_double_lock

Make use of the swap macro and remove unnecessary variable *tmp*.
This makes the code easier to read and maintain.

This code was detected with the help of Coccinelle.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>