commit bd648e3 ("mdadm: Remove klibc and uclibc support") removes
macro HAVE_NFTW/HAVE_FTW and uses libc header ftw.h. But it leaves the
codes in lib.c which let mdadm command call nftw defined in lib.c. It
needs to remove these codes.

This issue is introduced by MR https://gitlab.com/redhat/centos-stream/rpms/mdadm/-/merge_requests/45, mdadm version is mdadm-4.4-1.el10. And 4.4-1 hasn't been released, the build result is at https://brewweb.engineering.redhat.com/brew/taskinfo?taskID=67482273

The bug can be reproduced by:
mdadm -CR /dev/md0 --level raid5 --metadata=1.1 --chunk=32 --raid-disks 3
--size 10000 /dev/loop1 /dev/loop2 /dev/loop3
mdadm /dev/md0 --grow --chunk=64
mdadm: /dev/md0: cannot open component unknown