sh: __copy_user function can corrupt the stack in case of exception

The __copy_user function can corrupt the stack in the case of a
non-trivial length of data, and either of the first two move instructions
cause an exception. This is because the fixup for these two instructions
is mapped to the no_pop case, but these instructions execute after the
stack is pushed.

This change creates an explicit NO_POP exception mapping macro, and uses
it for the two instructions executed in the trivial case where no stack
pushes occur.

More information at ST Linux bugzilla:

	https://bugzilla.stlinux.com/show_bug.cgi?id=4824



Signed-off-by: Dylan Reid <dylan_reid@bose.com>
Signed-off-by: Stuart Menefy <stuart.menefy@st.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>