[ARM] 3310/1: add a comment about the possible __kuser_cmpxchg transient false

[ARM] 3310/1: add a comment about the possible __kuser_cmpxchg transient false
 negative

Patch from Nicolas Pitre

The pre ARMv5 implementation can be aborted if an exception occurs in
the middle of it.  Because of that, the ARMv6 implementation doesn't
re-attempt the operation on a failed strex either.  Let's make this
transient nature of such a false positive more explicit in the
definition.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>