f2fs: fix KMSAN uninit-value in extent_info usage

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: f2fs: fix KMSAN uninit-value in extent_info usage KMSAN reported a use of uninitialized value in `__is_extent_mergeable()` and `__is_back_mergeable()` via the read extent tree path. The root cause is that `get_read_extent_info()` only initializes three fields (`fofs`, `blk`, `len`) of `struct extent_info`, leaving the remaining fields uninitialized. This leads to undefined behavior when those fields are accessed later, especially during extent merging. Fix it by zero-initializing the `extent_info` struct before population.

References

• https://git.kernel.org/stable/c/08e8ab00a6d20d5544c932ee85a297d833895141
• https://git.kernel.org/stable/c/e68b751ec2b15d866967812c57cfdfc1eba6a269
• https://git.kernel.org/stable/c/dabfa3952c8e6bfe6414dbf32e8b6c5f349dc898
• https://git.kernel.org/stable/c/44a79437309e0ee2276ac17aaedc71253af253a8
• https://git.kernel.org/stable/c/cc1615d5aba4f396cf412579928539a2b124c8a0
• https://git.kernel.org/stable/c/01b6f5955e0008af6bc3a181310d2744bb349800
• https://git.kernel.org/stable/c/154467f4ad033473e5c903a03e7b9bca7df9a0fa