generic/486: adjust the max xattr size

generic/486: adjust the max xattr size

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.  And
since it reports 4M as the blocksize (the default ceph object size),
generic/486 will fail in ceph because the XATTR_SIZE_MAX value can't be used
in attr_replace_test.

The fix is to add a new argument to the test so that the max size can be
passed in instead of trying to auto-probe a value for it.

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