ceph: fix duplicate increment of opened_inodes metric

opened_inodes is incremented twice when the same inode is opened twice
with O_RDONLY and O_WRONLY respectively.

To reproduce, run this python script, then check the metrics:

import os
for _ in range(10000):
    fd_r = os.open('a', os.O_RDONLY)
    fd_w = os.open('a', os.O_WRONLY)
    os.close(fd_r)
    os.close(fd_w)

Fixes: 1dd8d470

 ("ceph: metrics for opened files, pinned caps and opened inodes")
Signed-off-by: Hu Weiwen <sehuww@mail.scut.edu.cn>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>