ext4: fix block bitmap validation when bigalloc, ^flex_bg

On a bigalloc,^flex_bg filesystem, the ext4_valid_block_bitmap
function fails to convert from blocks to clusters when spot-checking
the validity of the bitmap block that we've just read from disk.  This
causes ext4 to think that the bitmap is garbage, which results in the
block group being taken offline when it's not necessary.  Add in the
necessary EXT4_B2C() calls to perform the conversions.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>