befs: add flags field to validate superblock state

For validating superblock state, add flags field to befs_sb_info, read the state from the disk
and check if it is equal to BEFS_DIRTY.

Signed-off-by: Salah Triki <salah.triki@gmail.com>
Signed-off-by: Luis de Bethencourt <luisbg@osg.samsung.com>