f2fs: fix use-after-free of sbi in f2fs_compress_write_end_io()

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

f2fs: fix use-after-free of sbi in f2fs_compress_write_end_io()

In f2fs_compress_write_end_io(), dec_page_count(sbi, type) can bring

the F2FS_WB_CP_DATA counter to zero, unblocking

f2fs_wait_on_all_pages() in f2fs_put_super() on a concurrent unmount

CPU. The unmount path then proceeds to call

f2fs_destroy_page_array_cache(sbi), which destroys

sbi->page_array_slab via kmem_cache_destroy(), and eventually

kfree(sbi). Meanwhile, the bio completion callback is still executing:

when it reaches page_array_free(sbi, ...), it dereferences

sbi->page_array_slab — a destroyed slab cache — to call

kmem_cache_free(), causing a use-after-free.

This is the same class of bug as CVE-2026-23234 (which fixed the

equivalent race in f2fs_write_end_io() in data.c), but in the

compressed writeback completion path that was not covered by that fix.

Fix this by moving dec_page_count() to after page_array_free(), so

that all sbi accesses complete before the counter decrement that can

unblock unmount. For non-last folios (where atomic_dec_return on

cic->pending_pages is nonzero), dec_page_count is called immediately

before returning — page_array_free is not reached on this path, so

there is no post-decrement sbi access. For the last folio,

page_array_free runs while the F2FS_WB_CP_DATA counter is still

nonzero (this folio has not yet decremented it), keeping sbi alive,

and dec_page_count runs as the final operation.