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

Summary

In the Linux kernel, the following vulnerability has been resolved: 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 814b84971388 ("pNFS/NFSv4: Fix a layout segment leak in pnfs_layout_process()") added the drain; commit 08bd8dbe8882 ("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 fb700ef02676 ("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.