libcephfs: clarify interface return value

Document that ceph_get_stripe_unit_granularity may return an error code
(e.g. -ENOTCONN). The interface requires a mount, but currently we
return a compile-time constant. Other error codes may be possible in the
future.

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
libcephfs: return -ENOTCONN when call unmounted

Adds -ENOTCONN return value for stat, fchmod, fchown, lchown.

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
test: enforce -ENOTCONN contract in libcephfs

Tests all relevant calls for -ENOTCONN when used with an unmounted
ceph_mount_info param.

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>