UBI: Update comments to reflect UBI_METAONLY flag

This patch trivially updates code comments to reflect the addition of the
UBI_METAONLY flag - as discussed https://lkml.org/lkml/2014/10/29/764



CRs-Fixed: 975289
Change-Id: Ia44fe3def992e497e3a907b1a61ddfc63e07cbb5
Cc: Richard Weinberger <richard@nod.at>
Cc: trivial@kernel.org
Signed-off-by: Andrew Murray <amurray@embedded-bits.co.uk>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Git-commit: 061eebba3d3077186609da7d8029e0fd89f194e7
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Nikhilesh Reddy <reddyn@codeaurora.org>