luci-base: Avoid block umount on fstab apply

Default behaviour of changes to fstab (Mount Points) was
to use /etc/init.d/fstab restart, however this unmounts
filesystems via block umount which can cause the device
to fail, so replace the initscript call with an exec
of 'block mount'.

Signed-off-by: Daniel Dickinson <lede@cshore.thecshore.com>