uaccess: Add non-pagefault user-space read functions

Add probe_user_read(), strncpy_from_unsafe_user() and
strnlen_unsafe_user() which allows caller to access user-space
in IRQ context.

Current probe_kernel_read() and strncpy_from_unsafe() are
not available for user-space memory, because it sets
KERNEL_DS while accessing data. On some arch, user address
space and kernel address space can be co-exist, but others
can not. In that case, setting KERNEL_DS means given
address is treated as a kernel address space.
Also strnlen_user() is only available from user context since
it can sleep if pagefault is enabled.

To access user-space memory without pagefault, we need
these new functions which sets USER_DS while accessing
the data.

Link: http://lkml.kernel.org/r/155789869802.26965.4940338412595759063.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>