xattr: mark variable as uninitialized to make both gcc and smatch happy

new_xattr in __simple_xattr_set() is only initialized with a valid
pointer if value is not NULL, which only happens if this function is
called directly with the intention to remove an existing extended
attribute. Even being safe to be this way, smatch warns about possible
NULL dereference. Dan Carpenter suggested using uninitialized_var()
which will make both gcc and smatch happy.

Cc: Fengguang Wu <fengguang.wu@intel.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Aristeu Rozanski <aris@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>