These were fd 3, 4, and 5 by the time zfs change-key hit
execute_key_fob()
glibc appends "e" to setmntent() mode, but musl's just returns fopen()
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes#11866