Add xvattr support

Add xvattr support

With the removal of the minimal xvattr support from the spl this
support needs to be replaced in the zfs package.  This is fairly
easily accomplished by directly adding portions of the sys/vnode.h
header from OpenSolaris.  These xvattr additions have been placed
in the sys/xvattr.h header file and included as needed where simply
a sys/vnode.h was included before.

In additon to the xvattr types and helper macros two functions
were also included.  The xva_init() and xva_getxoptattr() functions
were included as static inline functions in xvattr.h.  They are
simple enough and it was simpler to place them here rather than
in their own .c file.