LU-508 ldiskfs: fix race in ext4_ext_walk_space()

LU-508 ldiskfs: fix race in ext4_ext_walk_space()

we should not access on-disk data (e.g. path->p_ext->*) with no
locking.

to be fixed in mainline ext4 as well.

Port from: ORI-291
Author: Alex Zhuravlev <bzzz@whamcloud.com>
Signed-off-by: Bobi Jam <bobijam@whamcloud.com>
Change-Id: Ic9dfcc9a90766c79c0da5ca54e9fbb2f917865a6
Reviewed-on: http://review.whamcloud.com/1618
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Mikhail Pershin <tappro@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>