client: fix _listxattr() vxattr buffer length calculation

client: fix _listxattr() vxattr buffer length calculation

Client::_listxattr() incorrectly returns a length based on the static
_vxattrs_name_size() value. _vxattrs_calcu_name_size() only takes into
account whether vxattrs are hidden, ignoring vxattr.exists_cb().

When filling the xattr buffer, Client::_listxattr() checks vxattr.hidden
and vxattr.exists_cb(). When the latter returns false (as is the case
for ceph.snap.btime on non-snapshots), we return a length which is
larger than the amount that we wrote to the buffer.

Fix this behaviour by always calculating the vxattrs length at runtime,
taking both vxattr.hidden and vxattr.exists_cb() into account.

Signed-off-by: David Disseldorp <ddiss@suse.de>