f2fs: avoid data offset overflow when lseeking huge file

xfstest generic/285 complains our issue in lseeking huge file.

Here is the detail output of generic/285:
"./check -f2fs tests/generic/285
Ran: generic/285
Failures: generic/285
Failed 1 of 1 tests

10. Test a huge file for offset overflow
10.01 SEEK_HOLE expected 65536 or 8589934592, got 65536.          succ
10.02 SEEK_HOLE expected 65536 or 8589934592, got 65536.          succ
10.03 SEEK_DATA expected 0 or 0, got 0.                           succ
10.04 SEEK_DATA expected 1 or 1, got 1.                           succ
10.05 SEEK_HOLE expected 8589934592 or 8589934592, got 0.         FAIL
10.06 SEEK_DATA expected 8589869056 or 8589869056, got 8589869056. succ
10.07 SEEK_DATA expected 8589869057 or 8589869057, got 8589869057. succ
10.08 SEEK_DATA expected 8589869056 or 8589869056, got 4294901760. FAIL"

The reason of this issue is:
We will calculate current offset through left shifting page-offset with
PAGE_CACHE_SHIFT bits, but our page-offset is a type of unsigned long, its size
is 4 bytes in 32-bits machine.

So if our page-offset is bigger than (1 << 32 / pagesize - 1), result of left
shifting will overflow.

Let's fix this issue by casting type of page-offset to type of current offset:
loff_t.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>