generic: add a test that stresses metadata eviction

generic: add a test that stresses metadata eviction

The fsverity test generic/579 could be used to reproduce a regression in
the xfs buffer hashing.  Carlos Maiolino figured out that even removing
the fsverity-specific bits would reproduce the issue.  This test is
thus a copy of generic/579 with the fsverity bits striped out, and
run 5 times as that seems to be the threshold at which the issue
reproduces reliably.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zorro Lang <zlang@kernel.org>