arm64: Rename get_thread_info()

The assembly macro get_thread_info() actually returns a task_struct and is
analogous to the current/get_current macro/function.

While it could be argued that thread_info sits at the start of
task_struct and the intention could have been to return a thread_info,
instances of loads from/stores to the address obtained from
get_thread_info() use offsets that are generated with
offsetof(struct task_struct, [...]).

Rename get_thread_info() to state it returns a task_struct.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Julien Thierry <julien.thierry@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>