SMACK: Fix handling value==NULL in post setxattr

The function `smack_inode_post_setxattr` is called each
time that a setxattr is done, for any value of name.
The kernel allow to put value==NULL when size==0
to set an empty attribute value. The systematic
call to smk_import_entry was causing the dereference
of a NULL pointer hence a KERNEL PANIC!

The problem can be produced easily by issuing the
command `setfattr -n user.data file` under bash prompt
when SMACK is active.

Moving the call to smk_import_entry as proposed by this
patch is correcting the behaviour because the function
smack_inode_post_setxattr is called for the SMACK's
attributes only if the function smack_inode_setxattr validated
the value and its size (what will not be the case when size==0).

It also has a benefical effect to not fill the smack hash
with garbage values coming from any extended attribute
write.

Change-Id: Iaf0039c2be9bccb6cee11c24a3b44d209101fe47
Signed-off-by: José Bollo <jose.bollo@open.eurogiciel.org>

security/smack/smack_lsm.c

@@ -960,18 +960,20 @@ static void smack_inode_post_setxattr(struct dentry *dentry, const char *name,
 		return;
 	}
 
-	skp = smk_import_entry(value, size);
 	if (strcmp(name, XATTR_NAME_SMACK) == 0) {
+		skp = smk_import_entry(value, size);
 		if (skp != NULL)
 			isp->smk_inode = skp->smk_known;
 		else
 			isp->smk_inode = smack_known_invalid.smk_known;
 	} else if (strcmp(name, XATTR_NAME_SMACKEXEC) == 0) {
+		skp = smk_import_entry(value, size);
 		if (skp != NULL)
 			isp->smk_task = skp;
 		else
 			isp->smk_task = &smack_known_invalid;
 	} else if (strcmp(name, XATTR_NAME_SMACKMMAP) == 0) {
+		skp = smk_import_entry(value, size);
 		if (skp != NULL)
 			isp->smk_mmap = skp;
 		else