f2fs: fix KMSAN uninit-value in extent_info usage

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.