maint: clarify df's use of device IDs from /proc/self/mountinfo

maint: clarify df's use of device IDs from /proc/self/mountinfo

* src/df.c (filter_mount_list): Clarify why we still stat even
though devices IDs may already be available.  Note using
/proc/self/mountinfo is still an advantage to get filtered items
with accurate device patchs in chroots and with bind mounts.
I.E. on older setups with static /etc/mtab, df will now
bypass that to get the more accuracte and dynamic info.