security/smack: fix oops when setting a size 0 SMACK64 xattr

upstream commit: 4303154e86597885bc3cbc178a48ccbc8213875f

this patch fix an oops in smack when setting a size 0 SMACK64 xattr eg
attr -S -s SMACK64  -V '' somefile
This oops because smk_import_entry treats a 0 length as SMK_MAXLEN

Signed-off-by: Etienne Basset <etienne.basset@numericable.fr>
Reviewed-by: James Morris <jmorris@namei.org>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>