btrfs: fix failure of RWF_NOWAIT write into prealloc extent beyond eof

If we attempt to write to prealloc extent located after eof using a
RWF_NOWAIT write, we always fail with -EAGAIN.

We do actually check if we have an allocated extent for the write at
the start of btrfs_file_write_iter() through a call to check_can_nocow(),
but later when we go into the actual direct IO write path we simply
return -EAGAIN if the write starts at or beyond EOF.

Trivial to reproduce:

  $ mkfs.btrfs -f /dev/sdb
  $ mount /dev/sdb /mnt

  $ touch /mnt/foo
  $ chattr +C /mnt/foo

  $ xfs_io -d -c "pwrite -S 0xab 0 64K" /mnt/foo
  wrote 65536/65536 bytes at offset 0
  64 KiB, 16 ops; 0.0004 sec (135.575 MiB/sec and 34707.1584 ops/sec)

  $ xfs_io -c "falloc -k 64K 1M" /mnt/foo

  $ xfs_io -d -c "pwrite -N -V 1 -S 0xfe -b 64K 64K 64K" /mnt/foo
  pwrite: Resource temporarily unavailable

On xfs and ext4 the write succeeds, as expected.

Fix this by removing the wrong check at btrfs_direct_IO().

Fixes: edf064e7 ("btrfs: nowait aio support")
CC: stable@vger.kernel.org # 4.14+
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>