Using asm_volatile_goto for get_user()

[David Laight]

I had an idea that might let 'asm_volatile_goto' be used for
get_user() even though gcc doesn't allow outputs.

What if (eg) 'register eax asm ("eax") is used for the
output and (probably) given in the 'clobber' list.

Such variables are usually used to get explicit registers
used when there is no suitable constraint.
I don't see why it shouldn't work for 'asm goto' as well
as just 'asm'.

While this forces the read value into a specific register
that probably doesn't make much difference to the code.

	David

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