erofs: move several xattr helpers into xattr.c

erofs: move several xattr helpers into xattr.c

Move xattrblock_addr() and xattrblock_offset() helpers into xattr.c,
as they are not used outside of xattr.c.

inlinexattr_header_size() has only one caller, and thus make it inlined
into the caller directly.

Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20230330082910.125374-2-jefflexu@linux.alibaba.com
Reviewed-by: Yue Hu <huyue2@coolpad.com>
Acked-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>