ext4: fix possible overflow in ext4_trim_fs()

When determining last group through ext4_get_group_no_and_offset() the
result may be wrong in cases when range->start and range-len are too
big, because it may overflow when summing up those two numbers.

Fix that by checking range->len and limit its value to
ext4_blocks_count(). This commit was tested by myself with expected
result.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>