getiobuf() allocates a
buf structure.
vp
The vnode to which the allocated buffer will be associated. This can be NULL.
waitok
If true, getiobuf can sleep until enough memory is available. Otherwise, it returns NULL immediately if enough memory is not available.
Note that the allocated buffer doesn't belong to buffer cache. To free it,
putiobuf() should be used.
brelse() should not be used on it.
putiobuf() frees
bp, which should be a buffer allocated with
getiobuf().