Relocate do_ftruncate/do_lseek/do_fallocate/do_punch_hole out of
syscall.c into vfs/fileio.c as the vfs_* names, declared in vfs/vfs.h.
The SUPPORT_PREALLOCATION / HAVE_FALLOCATE / HAVE_SYS_FALLOCATE /
FALLOC_FL_PUNCH_HOLE guards travel verbatim. This was the last
operation family: syscall.c no longer defines any filesystem wrapper.
No behavior change. (Portability-sensitive; wants a fleettest.)