btrfs: disable defrag on pre-content watched files

We queue up inodes to be defrag'ed asynchronously, which means we do not
have their original file for readahead.  This means that the code to
skip readahead on pre-content watched files will not run, and we could
potentially read in empty pages.

Handle this corner case by disabling defrag on files that are currently
being watched for pre-content events.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/4cc5bcea13db7904174353d08e85157356282a59.1731684329.git.josef@toxicpanda.com