ocfs2: fix data corruption on truncate

Patch series "ocfs2: Truncate data corruption fix".

As further testing has shown, commit 5314454e ("ocfs2: fix data
corruption after conversion from inline format") didn't fix all the data
corruption issues the customer started observing after 6dbf7bb5
("fs: Don't invalidate page buffers in block_write_full_page()") This
time I have tracked them down to two bugs in ocfs2 truncation code.

One bug (truncating page cache before clearing tail cluster and setting
i_size) could cause data corruption even before 6dbf7bb5, but before
that commit it needed a race with page fault, after 6dbf7bb5 it
started to be pretty deterministic.

Another bug (zeroing pages beyond old i_size) used to be harmless
inefficiency before commit 6dbf7bb5.  But after commit 6dbf7bb5
in combination with the first bug it resulted in deterministic data
corruption.

Although fixing only the first problem is needed to stop data
corruption, I've fixed both issues to make the cod...