f2fs: fix information leak in f2fs_move_inline_dirents()

In the Linux kernel, the following vulnerability has been resolved:

f2fs: fix information leak in f2fs_move_inline_dirents()

When converting an inline directory to a regular one, f2fs is leaking
uninitialized memory to disk because it doesn't initialize the entire
directory block. Fix this by zero-initializing the block.

This bug was introduced by commit 4ec17d688d74 ("f2fs: avoid unneeded
initializing when converting inline dentry"), which didn't consider the
security implications of leaking uninitialized memory to disk.

This was found by running xfstest generic/435 on a KMSAN-enabled kernel.