sys/sys/vnode.h: Introduce VOP_EIO macro for I/O errno in VOPs

We later make use of this to reduce some repetition of a
common helper VNOP function that return input/output error (EIO)
on 'bad' VOP's. As is defined in 'sys/sys/errno.h'.

Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
linprocfs_vnops.c: Convert to use VOP_EIO macro

Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
procfs/procfs_vnops.c: Convert to use VOP_EIO macro

Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
devfs/devfs_vnop.c: Convert to use VOP_EIO macro

Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>