ext4: convert from atomic_t to refcount_t on ext4_io_end->count

refcount_t type and corresponding API can protect refcounters from
accidental underflow and overflow and further use-after-free situations.

Signed-off-by: Xiyu Yang <xiyuyang19@fudan.edu.cn>
Signed-off-by: Xin Tan <tanxin.ctf@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/1626674355-55795-1-git-send-email-xiyuyang19@fudan.edu.cn


Signed-off-by: Theodore Ts'o <tytso@mit.edu>