x86/entry: Use get_user() for the fast 32-bit syscall EBP fetch

__do_fast_syscall_32() fetches the user's EBP from the stack slot the
vDSO stashed it in.  On 64-bit kernels it used __get_user(), as a
micro-optimization: the address is explicitly 32 bits, so it cannot be
out of range.

That optimization has turned into a pessimization.  On x86-64,
get_user() checks the address with a compare and a conditional move,
while __get_user() does an lfence instead.  Use get_user() on both
32-bit and 64-bit, which also removes the IS_ENABLED(CONFIG_X86_64)
special case.

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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