CVE-2025-68821: fuse: fix readahead reclaim deadlock

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

fuse: fix readahead reclaim deadlock

Commit e26ee4efbc79 ("fuse: allocate ff->releaseargs only if release is needed") skips allocating ff->releaseargs if the server does not implement open. However in doing so, fusepreparerelease() now skips grabbing the reference on the inode, which makes it possible for an inode to be evicted from the dcache while there are inflight readahead requests. This causes a deadlock if the server triggers reclaim while servicing the readahead request and reclaim attempts to evict the inode of the file being read ahead. Since the folio is locked during readahead, when reclaim evicts the fuse inode and fuseevictinode() attempts to remove all folios associated with the inode from the page cache (truncateinodepagesrange()), reclaim will block forever waiting for the lock since readahead cannot relinquish the lock because it is itself blocked in reclaim:

>> stacktrace(1504735) foliowaitbitcommon (mm/filemap.c:1308:4) foliolock (./include/linux/pagemap.h:1052:3) truncateinodepagesrange (mm/truncate.c:336:10) fuseevictinode (fs/fuse/inode.c:161:2) evict (fs/inode.c:704:3) dentryunlinkinode (fs/dcache.c:412:3) dentrykill (fs/dcache.c:615:3) shrinkkill (fs/dcache.c:1060:12) shrinkdentrylist (fs/dcache.c:1087:3) prunedcachesb (fs/dcache.c:1168:2) supercachescan (fs/super.c:221:10) doshrinkslab (mm/shrinker.c:435:9) shrinkslab (mm/shrinker.c:626:10) shrinknode (mm/vmscan.c:5951:2) shrinkzones (mm/vmscan.c:6195:3) dotrytofreepages (mm/vmscan.c:6257:3) doswappage (mm/memory.c:4136:11) handleptefault (mm/memory.c:5562:10) handlemmfault (mm/memory.c:5870:9) douseraddrfault (arch/x86/mm/fault.c:1338:10) handlepagefault (arch/x86/mm/fault.c:1481:3) excpagefault (arch/x86/mm/fault.c:1539:2) asmexcpagefault+0x22/0x27

Fix this deadlock by allocating ff->releaseargs and grabbing the reference on the inode when preparing the file for release even if the server does not implement open. The inode reference will be dropped when the last reference on the fuse file is dropped (see fusefileput() -> fusereleaseend()).