Evicting too many bytes from MFU metadata

Without updating 'm' we evict from MFU metadata all that we wanted
to evict from all metadata, including already evicted MRU metadata
('m' is the total amount of metadata we had at the beginning,
and 'w' is the total amount of metadata we want to have).

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Theera K. <tkittich@hotmail.com>
Closes #16521
Closes #16546