generic: test to run fsx eof pollution

generic: test to run fsx eof pollution

Filesystem regressions related to partial page zeroing can go
unnoticed for a decent amount of time. A recent example is the issue
of iomap zero range not handling dirty pagecache over unwritten
extents, which leads to wrong behavior on certain file extending
operations (i.e. truncate, write extension, etc.).

fsx does occasionally uncover these sorts of problems, but failures
can be rare and/or require longer running tests outside what is
typically run via full fstests regression runs. fsx now supports a
mode that injects post-eof data in order to explicitly test partial
eof zeroing behavior. This uncovers certain problems more quickly
and applies coverage more broadly across size changing operations.

Add a new test that runs an fsx instance (modeled after generic/127)
with eof pollution mode enabled. While the test is generic, it is
currently limited to XFS as that is currently the only known major
fs that does enough zeroing to satisfy the strict semantics expected
by fsx. The long term goal is to uncover and fix issues so more
filesystems can enable this test.

Signed-off-by: Brian Foster <bfoster@redhat.com>