nfsd: convert nfsd_net boolean flags to unsigned long flags word

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

nfsd: convert nfsd_net boolean flags to unsigned long flags word

nfsd_net contains several boolean fields that are accessed from
concurrent contexts without serialization. In particular,
nfsd4_end_grace() guards its drain path with a plain bool:

if (nn->grace_ended)
return;
nn->grace_ended = true;

The read and the write are independent, and nothing in struct
nfsd_net serializes them. At least two contexts can reach this
code with no lock held:

laundromat path
laundry_wq kworker
nfs4_laundromat()
nfsd4_end_grace()

RECLAIM_COMPLETE path
nfsd compound kthread
nfsd4_reclaim_complete()
inc_reclaim_complete()
nfsd4_end_grace()

Both callers can observe grace_ended == false on different CPUs,
both store true, and both proceed into nfsd4_record_grace_done(),
which invokes the active client_tracking_ops->grace_done callback.
For tracking ops that drain reclaim_str_hashtbl (legacy_tracking_ops
via nfsd4_recdir_purge_old, and the cld v1+ ops via
nfsd4_cld_grace_done), grace_done calls nfs4_release_reclaim(),
which walks every bucket of reclaim_str_hashtbl with no lock and
calls nfs4_remove_reclaim_record() (list_del + kfree) on each
entry. Two concurrent walkers corrupt the list and double-free
every nfs4_client_reclaim. A concurrent nfsd4_find_reclaim_client()
iterating the same bucket reads through freed memory.

A third call site exists in nfs4_state_start_net() on the
skip_grace startup path, but it runs under nfsd_mutex before any
client has connected and before the laundromat's first delayed
work fires, so it cannot race with the two callers above.

Replace the scattered boolean fields in nfsd_net with a single
unsigned long flags word and an enum nfsd_net_flag for the bit
positions. The grace_ended race is fixed by using
test_and_set_bit(), which is atomic on all architectures. The
remaining flags (grace_end_forced, in_grace, somebody_reclaimed,
track_reclaim_completes, nfsd_net_up, lockd_up) are converted to
use test_bit/set_bit/clear_bit for consistency. This avoids
sub-word cmpxchg issues on architectures like Hexagon that only
support word-sized atomic operations.