microblaze: evict the check for kernel_mode(regs) from do_notify_resume()

Only one caller hasn't done it in assembler - work_pending on !MMU.
Everything else can't reach do_notify_resume() if we are returning
to kernel mode, so move that check to that sole caller and make
do_notify_resume() reachable only when returning to userland.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>