Introduce function btrfs_is_name_len_valid.

The function compares parameter @name_len with item boundary then
returns true if name_len is valid.
Signed-off-by: Su Yue <suy.fnst@cn.fujitsu.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ s/btrfs_leaf_data/BTRFS_LEAF_DATA_OFFSET/ ]
Signed-off-by: David Sterba <dsterba@suse.com>