sendfile tests: Mount filesystems under the test workdir

Otherwise they can't be run in parallel as they share a mount point.

MFC after:	1 week

tests/sys/kern/sendfile_test.sh

@@ -37,7 +37,7 @@
 #
 
 MD_DEVS="md.devs"
-MNT=/mnt
+MNT=mnt
 FILE=$MNT/file
 HELPER="$(atf_get_srcdir)/sendfile_helper"
 BSIZE=4096
@@ -134,10 +134,11 @@ common_body_setup()
 {
 	us=$1
 
+	atf_check mkdir $MNT
 	atf_check -o ignore -e ignore newfs -b $BSIZE -U -j /dev/${us}
 	atf_check mount /dev/${us} $MNT
 	atf_check -e ignore dd if=/dev/zero of=$FILE bs=1m count=1
-	atf_check umount /mnt
+	atf_check umount $MNT
 
 	load_gnop
 	atf_check gnop create /dev/${us}