brk: make sys_brk() honor COMPAT_BRK when computing lower bound

brk: make sys_brk() honor COMPAT_BRK when computing lower bound

Fix a regression introduced by

commit 4cc6028d4040f95cdb590a87db478b42b8be0508
Author: Jiri Kosina <[email protected]>
Date:   Wed Feb 6 22:39:44 2008 +0100

    brk: check the lower bound properly

The check in sys_brk() on minimum value the brk might have must take
CONFIG_COMPAT_BRK setting into account.  When this option is turned on
(i.e.  we support ancient legacy binaries, e.g.  libc5-linked stuff), the
lower bound on brk value is mm->end_code, otherwise the brk start is
allowed to be arbitrarily shifted.

Signed-off-by: Jiri Kosina <[email protected]>
Tested-by: Geert Uytterhoeven <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>