arm64: uaccess: refactor __{get,put}_user

As a step towards implementing __{get,put}_kernel_nofault(), this patch
splits most user-memory specific logic out of __{get,put}_user(), with
the memory access and fault handling in new __{raw_get,put}_mem()
helpers.

For now the LDR/LDTR patching is left within the *get_mem() helpers, and
will be removed in a subsequent patch.

There should be no functional change as a result of this patch.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: James Morse <james.morse@arm.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20201202131558.39270-7-mark.rutland@arm.com


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>