btrfs: remove ordered extent check and wait during fallocate

For fallocate() we have this loop that checks if we have ordered extents
after locking the file range, and if so unlock the range, wait for ordered
extents, and retry until we don't find more ordered extents.

This logic was needed in the past because:

1) Direct IO writes within the i_size boundary did not take the inode's
   VFS lock. This was because that lock used to be a mutex, then some
   years ago it was switched to a rw semaphore (commit 9902af79
   ("parallel lookups: actual switch to rwsem")), and then btrfs was
   changed to take the VFS inode's lock in shared mode for writes that
   don't cross the i_size boundary (commit e9adabb9 ("btrfs: use
   shared lock for direct writes within EOF"));

2) We could race with memory mapped writes, because memory mapped writes
   don't acquire the inode's VFS lock. We don't have that race anymore,
   as we have a rw semaphore to synchronize memory mapped writes with
   fallocate (and reflinking too). That change happened with commit
   8d9b4a16 ("btrfs: exclude mmap from happening during all
   fallocate operations").

So stop looking for ordered extents after locking the file range when
doing a plain fallocate.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>