ext4: fix uninit block bitmap initialization when s_meta_first_bg is non-zero

(cherry picked from commit 8dadb198)

The number of old-style block group descriptor blocks is
s_meta_first_bg when the meta_bg feature flag is set.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>