x86 get_unmapped_area: Access mmap_legacy_base through mm_struct member

x86 get_unmapped_area: Access mmap_legacy_base through mm_struct member

This is the updated version of df54d6fa5427 ("x86 get_unmapped_area():
use proper mmap base for bottom-up direction") that only randomizes the
mmap base address once.

Signed-off-by: Radu Caragea <[email protected]>
Reported-and-tested-by: Jeff Shorey <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Michel Lespinasse <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Adrian Sendroiu <[email protected]>
Cc: Greg KH <[email protected]>
Cc: Kamal Mostafa <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>