[PATCH 0/2 v2] minor cleanups to EFLAGS initialisation in ret_from_fork

[Ian Campbell <Ian.Campbell@citrix.com>]

The following series removes the use of a global kernel_eflags variable
from the x86_64 ret_from_fork path and (very slightly) merges the 32 and
64 bit version of that code path.

kernel_eflags could be made a __read_mostly but actually there is no
reason to prefer the value at cpu_init() time to a compile time constant
value for the initial eflags after a fork.

Test booted on 32 and 64 bit.

changes since v1:
      * rebased onto tip/master
      * make 32 bit look like 64 bit instead of vice versa since the
        behaviour of resetting eflags with interrupts disabled before
        calling schedule_tail seems more obviously safe than resetting
        eflags with interrupts enabled afterwards (although I think both
        are actually correct).
      * As a consequence of the above patch 3/3 (use symbolic names for
        bits in eflags) goes away since we only use the MBO bit 1 now,
        which doesn't have a name.