MFC r331709: MFV r331708:
9321 arc_loan_compressed_buf() can increment arc_loaned_bytes by the wrong value

illumos/illumos-gate@9be12bd737714550277bd02b0c693db560976990

arc_loan_compressed_buf() increments arc_loaned_bytes by psize unconditionally
In the case of zfs_compressed_arc_enabled=0, when the buf is returned via
arc_return_buf(), if ARC_BUF_COMPRESSED(buf) is false, then arc_loaned_bytes
is decremented by lsize, not psize.

Switch to using arc_buf_size(buf), instead of psize, which will return
psize or lsize, depending on the result of ARC_BUF_COMPRESSED(buf).

Reviewed by: Matt Ahrens <matt@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Garrett D'Amore <garrett@damore.org>
Author: Allan Jude <allanjude@freebsd.org>