ZFS: Clean up cppcheck warnings where relevant, leaked memory etc

In a message from Wolfgang Denk highlighting warnings from cppcheck,
the patch will address those that are correctly diagnosed. Some are
false-positives:

> [fs/zfs/zfs.c:937]: (error) Memory leak: l
dmu_read() allocates "l" if successful, so error-case should not free
it.
> [fs/zfs/zfs.c:1141]: (error) Memory leak: dnbuf
dmu_read() allocates "dnbuf" if successful, so error-case should not
free it.
> [fs/zfs/zfs.c:1372]: (error) Memory leak: osp
zio_read() allocates "osp" if successful, so error-case should
not free it.
> [fs/zfs/zfs.c:1726]: (error) Memory leak: nvlist
int_zfs_fetch_nvlist() allocates "nvlist" if successful, so error-case
should not free it.

Signed-off-by: Jorgen Lundman <lundman@lundman.net>