Revert "mm/secretmem: use refcount_t instead of atomic_t"

Revert "mm/secretmem: use refcount_t instead of atomic_t"

This reverts commit 110860541f443f950c1274f217a1a3e298670a33.

Converting the "secretmem_users" counter to a refcount is incorrect,
because a refcount is special in zero and can't just be incremented (but
a count of users is not, and "no users" is actually perfectly valid and
not a sign of a free'd resource).

Reported-by: [email protected]
Cc: Jordy Zomer <[email protected]>
Cc: Kees Cook <[email protected]>,
Cc: Jordy Zomer <[email protected]>
Cc: James Bottomley <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>