ext4: use the nblocks arg to ext4_truncate_restart_trans()

nblocks is passed into ext4_truncate_restart_trans() from
ext4_ext_truncate_extend_restart() with a value different from the default
blocks_for_truncate(), but is being ignored.

The two other calls to ext4_truncate_restart_trans() already pass the
default value, which is then being recalculated inside the function.

Fix the problem by using the passed argument.

Signed-off-by: Amir Goldstein <amir73il@users.sf.net>