blk/kernel: explicit assign to fields in struct

blk/kernel: explicit assign to fields in struct

Clang on FreeBSD reports:
```
Building CXX object src/global/CMakeFiles/libglobal_objs.dir/pidfile.cc.o
../src/global/pidfile.cc:170:5: warning: ISO C++ requires field designators to be specified in declaration order; field 'l_whence' will be initialized after field 'l_start' [-Wreorder-init-list]
    .l_start = 0,
    ^~~~~~~~~~~~
../src/global/pidfile.cc:169:17: note: previous initialization for field 'l_whence' is here
    .l_whence = SEEK_SET,
                ^~~~~~~~
```
And Linux and BSD have different orders in their `struct flock`.
It also prevents the wrong initialisation on FreeBSD

Signed-off-by: Willem Jan Withagen <wjw@digiware.nl>