arm64: restore get_current() optimisation

arm64: restore get_current() optimisation

Commit c02433dd6de32f04 ("arm64: split thread_info from task stack")
inverted the relationship between get_current() and
current_thread_info(), with sp_el0 now holding the current task_struct
rather than the current thead_info. The new implementation of
get_current() prevents the compiler from being able to optimize repeated
calls to either, resulting in a noticeable penalty in some
microbenchmarks.

This patch restores the previous optimisation by implementing
get_current() in the same way as our old current_thread_info(), using a
non-volatile asm statement.

Acked-by: Will Deacon <[email protected]>
Signed-off-by: Mark Rutland <[email protected]>
Reported-by: Davidlohr Bueso <[email protected]>
Signed-off-by: Catalin Marinas <[email protected]>