CVE-2026-63817: f2fs: validate compress cache inode only when enabled

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

f2fs: validate compress cache inode only when enabled

F2FSCOMPRESSINO() uses NMI(sbi)->maxnid as the synthetic inode number for the compressed page cache inode. That inode only exists when the compresscache mount option is enabled.

When compresscache is disabled, maxnid is outside the valid inode range. A corrupted directory entry that points to ino == maxnid should therefore be rejected by f2fschecknidrange(). However, ismetaino() currently treats F2FSCOMPRESSINO() as a meta inode unconditionally, so f2fsiget() bypasses doreadinode() and its nid range check, and instantiates a fake internal inode instead.

Gate the compressed cache inode case on COMPRESSCACHE, matching f2fsinitcompressinode(). With compresscache disabled, ino == maxnid now follows the normal inode path and is rejected as an out-of-range nid.