x86: make the masked_user_access_begin() macro use its argument only once

x86: make the masked_user_access_begin() macro use its argument only once

This doesn't actually matter for any of the current users, but before
merging it mainline, make sure we don't have any surprising semantics.

We don't actually want to use an inline function here, because we want
to allow - but not require - const pointer arguments, and return them as
such.  But we already had a local auto-type variable, so let's just use
it to avoid any possible double evaluation.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>