ext4: Use ext4_discard_reservations instead of mballoc-specific call

In ext4_ext_truncate(), we should use the more generic
ext4_discard_reservations() call so we do the right thing when the
filesystem is mounted with the nomballoc option.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: Mingming Cao <cmm@us.ibm.com>