netfs: Remove outdated comments about prefaulting

I originally set out to make netfs_perform_write() behavior more
consistent with generic_perform_write(). However, netfs currently
treats a failure to make forward progress as a hard error and does not
retry where the generic code will loop around and retry.

Instead of a major code restructuring, just focus on improving the
comments.

The comment refers to a possible deadlock and to userspace address
checks. Neither of those things are a concern when using
copy_folio_from_iter_atomic() for atomic usercopies. It prevents
deadlocks by disabling page faults and it leverages user copy
functions that have their own access_ok() checks.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>