nfs: Fix incorrect engine registering for '--enghelp' list

nfs: Fix incorrect engine registering for '--enghelp' list

`ioengine` from `nfs` (internal) engine is incorrectly exported thus
overriding its value in constructor callbacks of other external engines,
that are used for registering engine for listing with `--enghelp`.

Because flist is unsafe to double adding it also making `engine_list` to
become corrupt and causing infinite loop or abnormal list termination
when printing engine list.

Issue: https://github.com/axboe/fio/issues/1655
Fixes: 9326926b ("NFS engine")
Signed-off-by: Vitaly Chikunov <vt@altlinux.org>