bcachefs: make bch2_target_to_text_sb static

The bch2_target_to_text_sb are not used outside the file disk_groups.c,
so the modification is defined as static.

fs/bcachefs/disk_groups.c:583:6: warning: no previous prototype for ‘bch2_target_to_text_sb’.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Closes: https://bugzilla.openanolis.cn/show_bug.cgi?id=7144


Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>