Btrfs: add missing cleanup on sysfs init failure

Btrfs: add missing cleanup on sysfs init failure

If we failed during initialization of sysfs, we weren't unregistering the
top level btrfs sysfs entry nor the debugfs stuff.
Not unregistering the top level sysfs entry makes future attempts to reload
the btrfs module impossible and the following is reported in dmesg:

[ 2246.451296] WARNING: CPU: 3 PID: 10999 at fs/sysfs/dir.c:486 sysfs_warn_dup+0x91/0xb0()
[ 2246.451298] sysfs: cannot create duplicate filename '/fs/btrfs'
[ 2246.451298] Modules linked in: btrfs(+) raid6_pq xor bnep rfcomm bluetooth binfmt_misc nfsd auth_rpcgss oid_registry nfs_acl nfs lockd fscache sunrpc parport_pc parport psmouse serio_raw pcspkr evbug i2c_piix4 e1000 floppy [last unloaded: btrfs]
[ 2246.451310] CPU: 3 PID: 10999 Comm: modprobe Tainted: G        W    3.13.0-fdm-btrfs-next-24+ #7
[ 2246.451311] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[ 2246.451312]  0000000000000009 ffff8800d353fa08 ffffffff816f1da6 0000000000000410
[ 2246.451314]  ffff8800d353fa58 ffff8800d353fa48 ffffffff8104a32c ffff88020821a290
[ 2246.451316]  ffff88020821a290 ffff88020821a290 ffff8802148f0000 ffff8800d353fb80
[ 2246.451318] Call Trace:
[ 2246.451322]  [<ffffffff816f1da6>] dump_stack+0x4e/0x68
[ 2246.451324]  [<ffffffff8104a32c>] warn_slowpath_common+0x8c/0xc0
[ 2246.451325]  [<ffffffff8104a416>] warn_slowpath_fmt+0x46/0x50
[ 2246.451328]  [<ffffffff81367dc5>] ? strlcat+0x65/0x90
(....)

This fixes the following change:

    btrfs: add simple debugfs interface
    commit 1bae30982bc86ab66d61ccb6e22792593b45d44d

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>