zfs ioctls: use fget_write / fget_read instead of getf wrapper for fget

This allows to ensure that we do not write to a file that was opened
for reading only or vice versa.

Also, use the correct capability in in zfs_ioc_send_new().

Differential Revision: https://reviews.freebsd.org/D2382
Reviewed by: delphij
MFC after: 17 days
Sponsored by: ClusterHQ