fuse: fix memory leak

kmemleak is reporting that 32 bytes are being leaked by FUSE:

unreferenced object 0xe373b270 (size 32):
comm "fusermount", pid 1207, jiffies 4294707026 (age 2675.187s)
hex dump (first 32 bytes):
01 00 00 00 00 00 00 00 01 00 00 00 00 00 00 00 ................
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
backtrace:
[<b05517d7>] kmemleak_alloc+0x27/0x50
[<b0196435>] kmem_cache_alloc+0xc5/0x180
[<b02455be>] fuse_alloc_forget+0x1e/0x20
[<b0245670>] fuse_alloc_inode+0xb0/0xd0
[<b01b1a8c>] alloc_inode+0x1c/0x80
[<b01b290f>] iget5_locked+0x8f/0x1a0
[<b0246022>] fuse_iget+0x72/0x1a0
[<b02461da>] fuse_get_root_inode+0x8a/0x90
[<b02465cf>] fuse_fill_super+0x3ef/0x590
[<b019e56f>] mount_nodev+0x3f/0x90
[<b0244e95>] fuse_mount+0x15/0x20
[<b019d1bc>] mount_fs+0x1c/0xc0
[<b01b5811>] vfs_kern_mount+0x41/0x90
[<b01b5af9>] do_kern_mount+0x39/0xd0
[<b01b7585>] do_mount+0x2e5/0x660
[<b01b7966>] sys_mount+0x66/0xa0

This leak report is consistent and happens once per boot on
3.1.0-rc5-dirty.

This happens if a FORGET request is queued after the fuse device was
released.

Reported-by: Sitsofe Wheeler <sitsofe@yahoo.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Tested-by: Sitsofe Wheeler <sitsofe@yahoo.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>