fspacectl(2)'s cmd parameter supports SPACECTL_ALLOC besides
SPACECTL_DEALLOC in this commit. posix_fallocate(2) is implemented with
fo_fspacectl under the hood.
VOP_ALLOCATE(9) is expanded to have an extra flags and cred parameters.
uipc_shm and NFS client/server are converted to use the new
struct fileops::fo_fallocate is eliminated.
Submitted by: Ka Ho Ng
Sponsored by: The FreeBSD Foundation