ARM: 9150/1: Fix PID_IN_CONTEXTIDR regression when THREAD_INFO_IN_TASK=y

The code that implements the rarely used PID_IN_CONTEXTIDR feature
dereferences the 'task' field of struct thread_info directly, and this
is no longer possible when THREAD_INFO_IN_TASK=y, as the 'task' field is
omitted from the struct definition in that case. Instead, we should just
cast the thread_info pointer to a task_struct pointer, given that the
former is now the first member of the latter.

So use a helper that abstracts this, and provide implementations for
both cases.

Reported by: Arnd Bergmann <arnd@arndb.de>

Fixes: 18ed1c01 ("ARM: smp: Enable THREAD_INFO_IN_TASK")
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>