x86/fpu: Use get_user()/put_user() for the signal frame fields

The FPU signal frame code reads and writes a handful of individual
fields (the xstate magic words, the legacy status word, the xfeatures
header and PKRU) with __get_user() and __put_user().  These rely on an
access_ok() of the whole frame done earlier by the caller.  Each of
those accesses enables and disables user access on its own, so the
up-front check saves nothing.  On x86, __get_user() is even more
expensive than get_user(): it does an lfence, where get_user() checks
the address with a couple of plain instructions.

Use get_user() and put_user().  The accesses that run with page faults
disabled stay that way; get_user() and put_user() are fine there.

This is part of getting rid of __get_user() and __put_user() entirely.

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

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