ceph: fix listxattr vxattr buffer length calculation

ceph_listxattr() incorrectly returns a length based on the static
ceph_vxattrs_name_size() value, which only takes into account whether
vxattrs are hidden, ignoring vxattr.exists_cb().

When filling the xattr buffer ceph_listxattr() checks VXATTR_FLAG_HIDDEN
and vxattr.exists_cb(). If both are false, we return an incorrect
(oversize) length.

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

This bug is only exposed with the new "ceph.snap.btime" vxattr, as all
other vxattrs with a non-null exists_cb also carry VXATTR_FLAG_HIDDEN.

Signed-off-by: David Disseldorp <ddiss@suse.de>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>