nilfs2: eliminate sparse warning - "context imbalance"

insert sparse annotations to fix following sparse warning.

fs/nilfs2/segment.c:2681:3: warning: context imbalance in 'nilfs_segctor_kill_thread' - unexpected unlock

nilfs_segctor_kill_thread is only called inside sc_state_lock lock.
sparse doesn't detect the context and warn "unexpected unlock".
__acquires/__releases pretend to lock/unlock the sc_state_lock for sparse.
Signed-off-by: NJiro SEKIBA <jir@unicus.jp>
Signed-off-by: NRyusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>

fs/nilfs2/segment.c

@@ -2674,6 +2674,8 @@ static int nilfs_segctor_start_thread(struct nilfs_sc_info *sci)
 }
 
 static void nilfs_segctor_kill_thread(struct nilfs_sc_info *sci)
+	__acquires(&sci->sc_state_lock)
+	__releases(&sci->sc_state_lock)
 {
 	sci->sc_state |= NILFS_SEGCTOR_QUIT;