btrfs: avoid blocking nowait dio when locking file range

If we are doing a NOWAIT direct IO read/write, we can block when locking
the file range at btrfs_dio_iomap_begin(), as it's possible the range (or
a part of it) is already locked by another task (mmap writes, another
direct IO read/write racing with us, fiemap, etc). We are also waiting for
completion of any ordered extent we find in the range, which also can
block us for a significant amount of time.

There's also the incorrect fallback to buffered IO (returning -ENOTBLK)
when we are dealing with a NOWAIT request and we can't proceed. In this
case we should be returning -EAGAIN, as falling back to buffered IO can
result in blocking for many different reasons, so that the caller can
delegate a retry to a context where blocking is more acceptable.

Fix these cases by:

1) Doing a try lock on the file range and failing with -EAGAIN if we
   can not lock right away;

2) Fail with -EAGAIN if we find an ordered extent;

3) Return -EAGAIN instead of -ENOTBLK when we need to fallback to
   buffered IO and we have a NOWAIT request.

This will also allow us to avoid a duplicated check that verifies if we
are able to do a NOCOW write for NOWAIT direct IO writes, done in the
next patch.

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