overlayfs: barriers for opening upper-layer directory
overlayfs: barriers for opening upper-layer directory

make sure that
	a) all stores done by opening struct file don't leak past storing
the reference in od->upperfile
	b) the lockless side has read dependency barrier

Signed-off-by: Al Viro <de609eb4d5d70b1d38ec6642adbfc33a2781f63c@zeniv.linux.org.uk>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs