ext4: fixup possible uninitialized variable access in ext4_mb_choose_next_group_cr1()

Variable 'grp' may be left uninitialized if there's no group with
suitable average fragment size (or larger). Fix the problem by
initializing it earlier.

Link: https://lore.kernel.org/r/20220922091542.pkhedytey7wzp5fi@quack3
Fixes: 83e80a6e

 ("ext4: use buckets for cr 1 block scan instead of rbtree")
Cc: stable@kernel.org
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>