vfs_ceph_new: refactor error-case in cephmount_mount_fs

vfs_ceph_new: refactor error-case in cephmount_mount_fs

Align code-style of 'cephmount_mount_fs' with rest of the code: use
'goto' for bail-out upon error case (with proper cleanups). For the
common case of successful operation complete execution and return final
value. Added extra debug-logging for good-path case.

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 d9b872afeee4dee49de2e6eb86e4b59e07804363)