Btrfs: fix off-by-one in cow_file_range_inline()

Btrfs could still inline file data if its size is same as
page size, so don't skip max value here.

Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>