xfs, iomap: fix data corrupton due to stale cached iomap

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2155605 Tested: With xfstests and bz reproducer

This series has as its main goal to fix a data corruption resulted of a race between unaligned buffered writes with low memory conditions causing both writeback and memory reclaim to race with the writes.

By fixing the race itself, a few other issues were uncovered, so the series not only fixes the race described above, but also a race in xfs itself, resulted from truncating page cache and punching delalloc extents without synchronization.

Signed-off-by: Carlos Maiolino cmaiolino@redhat.com