erofs-utils: lib: tar: fix LIBARCHIVE.xattr URL-encoded keys

erofs-utils: lib: tar: fix LIBARCHIVE.xattr URL-encoded keys

As tar(5) [1] mentioned: `The key value is URL-encoded: All non-ASCII
characters and the two special characters "=" and "%" are encoded as
"%" followed by two uppercase hexadecimal digits.`

Fix it now.

[1] https://man.freebsd.org/cgi/man.cgi?tar(5)

Fixes: c0063a73b01b ("erofs-utils: lib: support importing xattrs from tarerofs")
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20241226045808.95101-3-hsiangkao@linux.alibaba.com