os/chain_xattr: Remove all old xattr entry when overwrite the xattr.

Ceph use multiple xattrs to store the value of a single xattr which size
is larger than CHAIN_XATTR_MAX_BLOCK_LEN.
But when overwote the content of xattr in func
chain_setxattr/chain_fsetxattr, we don't know the size of previous
content of the xattr.
So we only try to remove until system return  -ENODATA.

Signed-off-by: Ma Jianpeng <jianpeng.ma@intel.com>