dma-buf: Cache the dma-buf inode number

Now that dma-buf has switched to handling its release from the
file_operations' release function to the dentry_operations' d_release
function there is a potential use-after-free when accessing the
buf_obj->file->f_inode as the path called by __fput eventually calls
__dentry_kill which frees the inode before calling the dentry_operations'
d_release function, so buf_obj->file->f_inode may no longer be valid.

It doesn't look like there is a clean way to check if the inode is valid.
For example using dentry->d_lock to look at the dentry inode would first
involve getting the dentry via a call to file_dentry which potentially
uses buf_obj->file->f_inode and it may no longer be valid.

It looks safer to simply cache the inode number since it is a useful value
to have as it identifies dma-buf objects.

KASAN log gave the following:
KASAN: use-after-free in dma_buf_debug_show+0x190/0x660
E : Read of size 8 at addr ffffff81051d7740 by task cat/19499
I Call trace:
I : dump_backtrace+0x0/0x2c8
I : show_stack+0x20/0x2c
I : dump_stack+0x154/0x1f4
I : print_address_description+0x84/0x580
I : __kasan_report+0x1b8/0x218
I : kasan_report+0x10/0x18
I : __asan_load8+0x90/0x98
I : dma_buf_debug_show+0x190/0x660
I : seq_read+0x2dc/0x830
I : full_proxy_read+0xbc/0x110
I : __vfs_read+0xd8/0x3d0
I : vfs_read+0x158/0x278
I : ksys_read+0x114/0x1e0
I : __arm64_sys_read+0x4c/0x5c
I : el0_svc_common+0x130/0x294
I : el0_svc_handler+0xdc/0xec
I : el0_svc+0x8/0xc
E : Allocated by task 912:
I : __kasan_kmalloc+0xfc/0x1b8
I : kasan_slab_alloc+0x14/0x1c
I : kmem_cache_alloc+0x320/0x3c4
I : new_inode_pseudo+0xbc/0x168
I : alloc_anon_inode+0x28/0x170
I : dma_buf_export+0x1e0/0x3ec
I : ion_dmabuf_alloc+0x168/0x1d4
I : ion_ioctl+0x24c/0x5c8
I : do_vfs_ioctl+0x55c/0xcb4
I : __arm64_sys_ioctl+0xa0/0xe4
I : el0_svc_common+0x130/0x294
I : el0_svc_handler+0xdc/0xec
I : el0_svc+0x8/0xc
E : Freed by task 40:
I : __kasan_slab_free+0x160/0x22c
I : kasan_slab_free+0x10/0x1c
I : slab_free_freelist_hook+0xdc/0x15c
I : kmem_cache_free+0xf8/0x350
I : i_callback+0x48/0x220
I : rcu_do_batch+0x2e8/0x594
I : nocb_cb_wait+0x158/0x930
I : rcu_nocb_cb_kthread+0x20/0x44
I : kthread+0x1ec/0x204

Change-Id: Ic3cfabf3524d2d34fe8b341e84f2ff259e4fad63
Signed-off-by: Liam Mark <lmark@codeaurora.org>