cephfs: Fixes for coverity bugs 716840 and 716970

cephfs:  Fixes for coverity bugs 716840 and 716970

Fixes for two coverity bugs in cephfs.cc.

716840 (CHECKED_RETURN) reported that the return value from
fstat wasn't getting checked.  It now is checked, reports an
error and returns to the caller.

716970 (NEGATIVE_RETURNS) reported that the file descriptor passed
to fstat (*fd) might be negative, which isn't allowed.  The check at
the top that open failed wasn't aborting the function (when *fd < 0),
now it does.

Signed-off-by: Sam Lang <sam.lang@inktank.com>