NFSv4.1: fix layout segment leak on the pnfs_layout_process() forget path

When the server returns a new layout stateid while a valid one is still
held, pnfs_layout_process() calls pnfs_mark_matching_lsegs_return() on
the on-stack free_me list and jumps to out_forget. Segments whose
reference count drops to zero are unlinked from lo->plh_segs and moved
to free_me by mark_lseg_invalid(); for an idle cached segment the layout
header holds the only reference, so this happens on the first decrement.

out_forget never drains free_me -- only the success path calls
pnfs_free_lseg_list().

Commit 814b849713 ("pNFS/NFSv4: Fix a layout segment leak in
pnfs_layout_process()") added the drain; commit 08bd8dbe88
("pNFS/NFSv4: Try to return invalid layout in pnfs_layout_process()")
removed it while switching the destination to lo->plh_return_segs, which
is drained elsewhere. Commit fb700ef026 ("NFSv4.1: Simplify layout
return in pnfs_layout_process()") switched the destination back to
free_me without restoring the drain.

Restore the pnfs_free_lseg_list() call.

Fixes: fb700ef026 ("NFSv4.1: Simplify layout return in pnfs_layout_process()")
Reported-by: Yuhao Jiang <danisjiang@gmail.com>
Assisted-by: Claude:claude-opus-5
Cc: stable@vger.kernel.org
Signed-off-by: Junrui Luo <moonafterrain@outlook.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/pnfs.c

@@ -2638,6 +2638,7 @@ pnfs_layout_process(struct nfs4_layoutget *lgp)
 	spin_unlock(&ino->i_lock);
 	lseg->pls_layout = lo;
 	NFS_SERVER(ino)->pnfs_curr_ld->free_lseg(lseg);
+	pnfs_free_lseg_list(&free_me);
 	return ERR_PTR(-EAGAIN);
 }