OpenZFS 9321 - arc_loan_compressed_buf() can increment arc_loaned_bytes by the wrong value

Authored by: Allan Jude <allanjude@freebsd.org>
Reviewed by: Matt Ahrens <matt@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Approved by: Garrett D'Amore <garrett@damore.org>
Ported-by: Giuseppe Di Natale <dinatale2@llnl.gov>

OpenZFS-issue: https://www.illumos.org/issues/9321
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/92b05f3a18
Closes #7333