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" <[e-mail]>
Reviewed-by: Mingming Cao <[e-mail]>

fs/ext4/extents.c

···
 	down_write(&EXT4_I(inode)->i_data_sem);
 	ext4_ext_invalidate_cache(inode);
 
-	ext4_mb_discard_inode_preallocations(inode);
+	ext4_discard_reservation(inode);
 
 	/*
 	 * TODO: optimization is possible here.