REQ_META set during meta data write

Today, storage devices like eMMC has special features like data tagging
(introduced in MMC-4.5 version) in order to improve performance of some
specific writes. On MMC stack, data tagging is used for all writes which has
REQ_META flag set. On EXT4, however, currently REQ_META is set only for read.
This patch adds the capability to add REQ_META flag during meta data write.
This patch adds new function set_buffer_meta, which is used to set a bit
BH_Meta in the b_state field when meta data and super block is made dirty.

Signed-off-by: Saugata Das <saugata.das@linaro.org>
Acked-by: Arnd Bergmann <arnd.bergmann@linaro.org>

Changelog v2:
	- Removed check for WRITE when setting REQ_META in submit_bh
Changelog v1:
	- Introduction of REQ_META during write
anykernel