ext4: fix test ext_generic_write_end() copied return value

'copied' is unsigned, whereas 'ret2' is not. The test (copied < 0) fails

Signed-off-by: Roel Kluin <12o3l@tiscali.nl>
Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>