ceph-handler: replace fuser by /proc/net/unix

ceph-handler: replace fuser by /proc/net/unix

We're using fuser command to see if a process is using a ceph unix
socket file. But the fuser command runs through every PID present in
/proc/<PID> to see if one of them is using the file.
On a system running thousands processes, the fuser command can take
a long time to finish.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1717011

Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>