devfs - Introduce MPSAFE read/write/ioctl support
* Add D_MPSAFE_READ, D_MPSAFE_WRITE and D_MPSAFE_IOCTL defines for use
with the dev_ops structure to mark the read,write and/or ioctl methods
as mpsafe.
* Change devfs so that it won't acquire the mplock for
read/writes/ioctls that are marked as MPSAFE.
* Add some statistics under sysctl vfs.devfs about the number of read
and writes done with the mplock and without it.
* While here, MPSAFE specf_stat in devfs so it doesn't acquire the
mplock anymore as it doesn't even need it.