f2fs: fix several potential integer overflows in file offsets

When dealing with large extents and calculating file offsets by
summing up according extent offsets and lengths of unsigned int type,
one may encounter possible integer overflow if the values are
big enough.

Prevent this from happening by expanding one of the addends to
(pgoff_t) type.

Found by Linux Verification Center (linuxtesting.org) with static
analysis tool SVACE.

Fixes: d323d005 ("f2fs: support file defragment")
Cc: stable@vger.kernel.org
Signed-off-by: Nikita Zhandarovich <n.zhandarovich@fintech.ru>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>