Consolidate arc_buf allocation checks

The following check currently occurs in three separate locations
in dbuf.c. This change consolidates those checks in to the
dbuf_alloc_arcbuf_from_arcbuf() function.

if (arc_is_encrypted(data)) {
...
} else if (compress_type != ZIO_COMPRESS_OFF) {
...
} else {
...
}

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #10057