libcephfs: switch to CEPHFS_E* errors

libcephfs: switch to CEPHFS_E* errors

The cephfs client errors have been switched from errno values
(OS specific) to CEPHFS_E* errors (based on the Linux codes) [1][2].

The issue is that libcephfs still uses errno, which means that
non-Linux clients will get inconsistent errors.

This change updates libcephfs to use CEPHFS_E* errors and stay
consistent with the underlying client library.

[1] https://github.com/ceph/ceph/blob/6ef176b6745199224b62482a1241752826a5be14/src/libcephfs.cc#L203
[2] https://tracker.ceph.com/issues/48802

Signed-off-by: Lucian Petrut <lpetrut@cloudbasesolutions.com>