vfs_ceph_new: improve mount cache-entry add

vfs_ceph_new: improve mount cache-entry add

Use boolean return value from cephmount_cache_add, to align code-style
with other caphmount helper functions. Returns false in case of memory
allocation failure, true otherwise (success).

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 866b872cdb2b08a8b5e6a9015cde9b34c4bcdf01)