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

include/sys/zfs_vnops.h

@@ -39,4 +39,17 @@ extern int mappedread(znode_t *, int, uio_t *);
 extern int mappedread_sf(znode_t *, int, uio_t *);
 extern void update_pages(znode_t *, int64_t, int, objset_t *, uint64_t);
 
+/*
+ * Platform code that asynchronously drops zp's inode / vnode_t.
+ *
+ * Asynchronous dropping ensures that the caller will never drop the
+ * last reference on an inode / vnode_t in the current context.
+ * Doing so while holding open a tx could result in a deadlock if
+ * the platform calls into filesystem again in the implementation
+ * of inode / vnode_t dropping (e.g. call from iput_final()).
+ */
+extern void zfs_zrele_async(znode_t *zp);
+
+extern zil_get_data_t zfs_get_data;
+
 #endif