gfs2: Minor gfs2_alloc_inode cleanup

In gfs2_alloc_inode, when kmem_cache_alloc cannot allocate a new object, return
NULL immediately.  The code currently relies on the fact that i_inode is the
first member in struct gfs2_inode and so ip and &ip->i_inode evaluate to the
same address, but that isn't immediately obvious.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Bob Peterson <rpeterso@redhat.com>