Reorder checks in the read/write(2) family of syscalls to prepare making

Reorder checks in the read/write(2) family of syscalls to prepare making
file operations mp-safe.

This change makes it clear that `f_offset' is only accessed in vn_read()
and vn_write(), which will help taking it out of the KERNEL_LOCK().

This refactoring uncovered a race in vn_read() which is now documented
and will be addressed in a later diff.

ok visa@