Decreases harm from races on VFS file offsets

Two syscalls could read, write, seek, or whatever on the same struct
file at a time.  In doing so, they race on the state of the file pointer
(offset).  The kernel could get confused slightly when figuring out
things like the page indices or the offsets.

Other parts of the kernel might catch the problem, like the page cache
or the memcpy, but better safe than sorry.