Add kern_sync(9).

[trasz]

Summary

Add kern_sync(9), and make kernel code call it instead of going
via sys_sync(2). Minor cleanup, no functional changes.

[kib]

For first commit it is fine, but I would prefer for kern_sync() to have more correct interface, e.g. it can return first error from VFS_SYNC() call (but still calling VFS_SYNC for all mount points).

sys/kern/vfs_syscalls.c

155

Arguably uap wants the __unused attribute.