UPSTREAM: arm: if we get into work_pending while returning to kernel mode, just go away

(cherry pick from commit 21c1176a)

checking in do_signal() is pointless - if we get there with !user_mode(regs)
(and we might), we'll end up looping indefinitely.  Check in work_pending
and break out of the loop if so.

Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Bug: 9674955
Bug: 25773781
Change-Id: Ia99f8631f2c4edb81bbd8f0af9a0a31614bc4c74