Remove an inappropriate crit_exit() in ehci.c and add a missing crit_exit()

Remove an inappropriate crit_exit() in ehci.c and add a missing crit_exit()
in kern/vfs_subr.c.  Specify string IDs in vfsync_bp() so we don't get
complaints on the console when the kernel is compiled with
DEBUG_CRIT_SECTIONS.

The missing crit_exit() in kern/vfs_subr.c was causing the kernel to leave
threads in a critical section, causing interrupts to stop operating and
cpu-bound userland programs to lock up the rest of the system.

Reported-by: Sascha Wildner <saw@online.de>, others.