x86/uaccess: Allow access_ok() in irq context if pagefault_disabled

x86/uaccess: Allow access_ok() in irq context if pagefault_disabled

WARN_ON_IN_IRQ() assumes that the access_ok() and following
user memory access can sleep. But this assumption is not
always correct; when the pagefault is disabled, following
memory access will just returns -EFAULT and never sleep.

Add pagefault_disabled() check in WARN_ON_ONCE() so that
it can ignore the case we call it with disabling pagefault.
For this purpose, this modified pagefault_disabled() as
an inline function.

Link: http://lkml.kernel.org/r/155789868664.26965.7932665824135793317.stgit@devnote2
Acked-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>