befs: add check for ag_shift in superblock

ag_shift and blocks_per_ag contain the same information in different ways,
same as block_shift and block_size do. It is worth checking this two are
consistent, but since blocks_per_ag isn't documented as mandatory to use
some implementations of befs don't enforce this, so making it non-fatal if
they don't match and just having it as a warning.

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