vfs_ceph_new: improve mount cache-entry ref-count

vfs_ceph_new: improve mount cache-entry ref-count

Use singed int32_t for cached mount-entries reference counting. Define
helper function for inc/dec ref-count which also provides proper
logging. Prefer boolean return-value for 'cephmount_cache_remove' as
'int' is often used as error indicator within the context of libcephfs
and this VFS module.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15703

Signed-off-by: Shachar Sharon <ssharon@redhat.com>
Reviewed-by: Anoop C S <anoopcs@samba.org>
Reviewed-by: John Mulligan <jmulligan@redhat.com>
(cherry picked from commit 29a9d18d2d21842bb38bcdc6b9e366abac458ed9)