btrfs: preallocate inodes xarray entry to avoid transaction abort

When creating a new inode, at btrfs_create_new_inode(), one of the very
last steps is to add the inode to the root's inodes xarray. This often
requires allocating memory which may fail (even though xarrays have a
dedicated kmem_cache which make it less likely to fail), and at that point
we are forced to abort the current transaction (as some, but not all, of
the inode metadata was added to its subvolume btree).

To avoid a transaction abort, preallocate memory for the xarray early at
btrfs_create_new_inode(), so that if we fail we don't need to abort the
transaction and the insertion into the xarray is guaranteed to succeed.

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>