gfs2: remove IS_ERR_VALUE abuse

Picked from commit 287980e4 ("remove lots
of IS_ERR_VALUE abuses") upstream.

The original fix that was backported to 3.18 already addressed the warning
in some configurations, but not in others, leaving us with the same output:

../fs/gfs2/dir.c: In function 'get_first_leaf':
../fs/gfs2/dir.c:768:9: warning: 'leaf_no' may be used uninitialized in this function [-Wmaybe-uninitialized]
   error = get_leaf(dip, leaf_no, bh_out);
         ^
../fs/gfs2/dir.c: In function 'dir_split_leaf.isra.20':
../fs/gfs2/dir.c:987:8: warning: 'leaf_no' may be used uninitialized in this function [-Wmaybe-uninitialized]

This takes the approach that we took in later versions in mainline,
but does not backport the entire patch, as that would be too large
for stable and IIRC caused regressions in other drivers.

Fixes: 9d46d31e ("gfs2: avoid uninitialized variable warning")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>