btrfs: simplify error check condition at btrfs_dirty_inode()

The following condition at btrfs_dirty_inode() is redundant:

  if (ret && (ret == -ENOSPC || ret == -EDQUOT))

The first check for a non-zero 'ret' value is pointless, we can simplify
this to simply:

  if (ret == -ENOSPC || ret == -EDQUOT)

Not only this makes it easier to read, it also slightly reduces the text
size of the btrfs kernel module:

  $ size fs/btrfs/btrfs.ko.before
     text	   data	    bss	    dec	    hex	filename
  1641400	 168265	  16864	1826529	 1bdee1	fs/btrfs/btrfs.ko.before

  $ size fs/btrfs/btrfs.ko.after
     text	   data	    bss	    dec	    hex	filename
  1641224	 168181	  16864	1826269	 1bdddd	fs/btrfs/btrfs.ko.after

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