zfs_vnops: make zfs_get_data OS-independent

zfs_vnops: make zfs_get_data OS-independent

Move zfs_get_data() in to platform-independent code. The only
platform-specific aspect of it is the way we release an inode
(Linux) / vnode_t (FreeBSD). I am not aware of a platform that
could be supported by ZFS that couldn't implement zfs_rele_async
itself. It's sibling zvol_get_data already is platform-independent.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Christian Schwarz <me@cschwarz.com>
Closes #10979