ext4: fix type declaration of ext4_validate_block_bitmap

The block_group parameter to ext4_validate_block_bitmap is both used
as a ext4_group_t inside the function and the same type is passed in
by all callers.  We might as well use the typedef consistently instead
of open-coding the 'unsigned int'.

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