Btrfs: fix regressions in copy_from_user handling

Commit 914ee295af418e936ec20a08c1663eaabe4cd07a fixed deadlocks in
btrfs_file_write where we would catch page faults on pages we had
locked.

But, there were a few problems:

1) The x86-32 iov_iter_copy_from_user_atomic code always fails to copy
data when the amount to copy is more than 4K and the offset to start
copying from is not page aligned.  The result was btrfs_file_write
looping forever retrying the iov_iter_copy_from_user_atomic

We deal with this by changing btrfs_file_write to drop down to single
page copies when iov_iter_copy_from_user_atomic starts returning failure.

2) The btrfs_file_write code was leaking delalloc reservations when
iov_iter_copy_from_user_atomic returned zero.  The looping above would
result in the entire filesystem running out of delalloc reservations and
constantly trying to flush things to disk.

3) btrfs_file_write will lock down page cache pages, make sure
any writeback is finished, do the copy_from_user and then release them.
Before the loop runs we check the first and last pages in the write to
see if they are only being partially modified.  If the start or end of
the write isn't aligned, we make sure the corresponding pages are
up to date so that we don't introduce garbage into the file.

With the copy_from_user changes, we're allowing the VM to reclaim the
pages after a partial update from copy_from_user, but we're not
making sure the page cache page is up to date when we loop around to
resume the write.

We deal with this by pushing the up to date checks down into the page
prep code.  This fits better with how the rest of file_write works.

Signed-off-by: Chris Mason <chris.mason@oracle.com>
Reported-by: Mitch Harder <mitch.harder@sabayonlinux.org>
cc: stable@kernel.org

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