If fd is unref'd at the end of async call then the unref in cbks would
lead to double unref and possible crash. Removing duplicate unrefs.

Added unref only in failure cases.

A simple test case has been added to test async write case. Need to
extend the same for other async APIs too.

Details:
All glfd based calls in libgfapi, except for glfs_open and glfs_close,
behave in the same way. At the start of the operation, they take a ref
on glfd and fd. At the end of the operation, they unref it. Async calls
are a little different as they unref in the cbk function. A successfull
open call does not unref either the glfd or fd, thereby functioning as a
reference for a OPEN file object. glfs_close makes a syncop_flush call
sandwiched between a fd ref and unref(this can be removed, more on this
below), followed by a call to glfs_mark_glfd_for_deletion which unrefs
glfd and also calls glfs_fd_destroy as a release function thereby doing
a unref on fd too.

Functionally, there is no problem with how everything works when as
described above. However, it is a little non-intuitive that we need to
perform a fd_unref as a consequence of a implicit fd_ref that happens
within glfs_resolve_fd. As we perform a GF_REF_GET(glfd) at the start of
every operation, it would be worthwhile to remove the fd_ref that
glfs_resovle_fd takes and do away with explicit fd_unref()s at the end
of every operation. This is the same reason why we don't need the fd_ref
in glfs_close. This is however not in the scope of this patch.

Change-Id: I86b1d3b2ad846b16ea527d541dc82b5e90b0ba85
BUG: 1391086
Signed-off-by: Raghavendra Talur <rtalur@redhat.com>
Reviewed-on: http://review.gluster.org/15768
Smoke: Gluster Build System <jenkins@build.gluster.org>
NetBSD-regression: NetBSD Build System <jenkins@build.gluster.org>
Reviewed-by: Rajesh Joseph <rjoseph@redhat.com>
Reviewed-by: Xavier Hernandez <xhernandez@datalab.es>
Reviewed-by: soumya k <skoduri@redhat.com>
Reviewed-by: Prasanna Kumar Kalever <pkalever@redhat.com>
CentOS-regression: Gluster Build System <jenkins@build.gluster.org>
Reviewed-by: Shyamsundar Ranganathan <srangana@redhat.com>