The function btrfs_check_shared() is supposed to be used to check if a
data extent is shared, but its name is too generic, may easily cause
confusion in the sense that it may be used for metadata extents.

So rename it to btrfs_is_data_extent_shared(), which will also make it
less confusing after the next change that adds a backref lookup cache for
the b+tree nodes that lead to the leaf that contains the file extent item
that points to the target data extent.
Reviewed-by: NJosef Bacik <josef@toxicpanda.com>
Reviewed-by: NQu Wenruo <wqu@suse.com>
Signed-off-by: NFilipe Manana <fdmanana@suse.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>