erofs-utils: introduce shared xattr support

Large xattrs or xattrs shared by a lot of files
can be stored in shared xattrs rather than
inlined right after inode.

Link: https://lore.kernel.org/r/20191014235308.4277-1-hsiangkao@aol.com
Signed-off-by: Li Guifu <blucerlee@gmail.com>
Signed-off-by: Gao Xiang <hsiangkao@aol.com>