linux: libzfs: util: don't fallthrough to to end-of-switch

lib/libzfs/os/linux/libzfs_util_os.c:262:3: error: fallthrough
annotation does not directly precede switch label

zfs_fallthrough;
^

./lib/libspl/include/sys/feature_tests.h:34:26: note: expanded from
macro 'zfs_fallthrough'

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13348