super: convert s_count to refcount_t s_passive

super: convert s_count to refcount_t s_passive

The superblock carries two counters: s_active, the active reference
count that keeps the filesystem usable, and s_count, the passive
reference count that merely keeps the structure itself alive. Turn the
passive count into a refcount_t and rename it to s_passive to make the
pairing with s_active obvious.

Everything is still serialized by sb_lock, so there is no functional
change; the conversion buys the usual refcount_t saturation and
underflow checking. The following patches start dropping passive
references without holding sb_lock and make the device-to-superblock
table hold one passive reference per registered entry, which a plain
integer cannot support.

Link: https://patch.msgid.link/20260616-work-super-bdev_holder_global-v2-2-7df6b864028e@kernel.org
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>