ANDROID: fuse-bpf: Check inode not null

fuse_iget_backing returns an inode or null, not a ERR_PTR. So check it's
not NULL

Also make sure we put the inode if d_splice_alias fails

Bug: 293349757
Bug: 300349748
Test: fuse_test runs
Signed_off_by: Paul Lawrence <paullawrence@google.com>

Change-Id: I1eadad32f80bab6730e461412b4b7ab4d6c56bf2
Signed-off-by: chenxin <chenxinxin@xiaomi.corp-partner.google.com>