vfs_ceph: improve readability of cephwrap_realpath

vfs_ceph: improve readability of cephwrap_realpath

Minor code cleanup: use local variable to reference current-work dir.
This commit is in preparation to following code-cleanup which aligns
line-length with Samba's coding conventions.

Signed-off-by: Shachar Sharon <ssharon@redhat.com>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: David Disseldorp <ddiss@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Guenther Deschner <gd@samba.org>
Reviewed-by: Anoop C S <anoopcs@samba.org>