ovl: support multiple lower layers

Allow "lowerdir=" option to contain multiple lower directories separated by
a colon (e.g. "lowerdir=/bin:/usr/bin").  Colon characters in filenames can
be escaped with a backslash.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

diff --cc Documentation/filesystems/overlayfs.txt
Simple merge

diff --cc fs/overlayfs/super.c
Simple merge