LU-7220 llog: fix fd lost and correct error reporting

LU-7220 llog: fix fd lost and correct error reporting

A file descriptor acquired to 'fd' may be lost in case of
error in is_fstype_ext() function.

Fix error code of is_fstype_ext() propagation.

Change-Id: I0894cfad83854c503e2cdcfff78d44f6f2746ef3
Signed-off-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-on: http://review.whamcloud.com/19407
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Jenkins
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>