block/gluster: prevent data loss after i/o error

Upon receiving an I/O error after an fsync, by default gluster will
dump its cache.  However, QEMU will retry the fsync, which is especially
useful when encountering errors such as ENOSPC when using the werror=stop
option.  When using caching with gluster, however, the last written data
will be lost upon encountering ENOSPC.  Using the write-behind-cache
xlator option of 'resync-failed-syncs-after-fsync' should cause gluster
to retain the cached data after a failed fsync, so that ENOSPC and other
transient errors are recoverable.

Unfortunately, we have no way of knowing if the
'resync-failed-syncs-after-fsync' xlator option is supported, so for now
close the fd and set the BDS driver to NULL upon fsync error.

Signed-off-by: Jeff Cody <jcody@redhat.com>