generic/020: adjust max_attrval_size for ceph

generic/020: adjust max_attrval_size for ceph

CephFS doesn't have a maximum xattr size.  Instead, it imposes a maximum
size for the full set of xattrs names+values, which by default is 64K.

This patch fixes the max_attrval_size for ceph so that it is takes into
account any already existing attrs in the file.

Signed-off-by: Luís Henriques <lhenriques@suse.de>
Reviewed-by: David Disseldorp <ddiss@suse.de>
Signed-off-by: Zorro Lang <zlang@kernel.org>