ceph: avoid accessing invalid memory

when mounting ceph with a dev name that starts with a slash, ceph
would attempt to access the character before that slash. Since we
don't actually own that byte of memory, we would trigger an
invalid access:

[   43.499934] BUG: unable to handle kernel paging request at ffff880fa3a97fff
[   43.500984] IP: [<ffffffff818f3884>] parse_mount_options+0x1a4/0x300
[   43.501491] PGD 743b067 PUD 10283c4067 PMD 10282a6067 PTE 8000000fa3a97060
[   43.502301] Oops: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
[   43.503006] Dumping ftrace buffer:
[   43.503596]    (ftrace buffer empty)
[   43.504046] CPU: 0 PID: 10879 Comm: mount Tainted: G        W    3.10.0-sasha #1129
[   43.504851] task: ffff880fa625b000 ti: ffff880fa3412000 task.ti: ffff880fa3412000
[   43.505608] RIP: 0010:[<ffffffff818f3884>]  [<ffffffff818f3884>] parse_mount_options$
[   43.506552] RSP: 0018:ffff880fa3413d08  EFLAGS: 00010286
[   43.507133] RAX: ffff880fa3a98000 RBX: ffff880fa3a98000 RCX: 0000000000000000
[   43.507893] RDX: ffff880fa3a98001 RSI: 000000000000002f RDI: ffff880fa3a98000
[   43.508610] RBP: ffff880fa3413d58 R08: 0000000000001f99 R09: ffff880fa3fe64c0
[   43.509426] R10: ffff880fa3413d98 R11: ffff880fa38710d8 R12: ffff880fa3413da0
[   43.509792] R13: ffff880fa3a97fff R14: 0000000000000000 R15: ffff880fa3413d90
[   43.509792] FS:  00007fa9c48757e0(0000) GS:ffff880fd2600000(0000) knlGS:000000000000$
[   43.509792] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[   43.509792] CR2: ffff880fa3a97fff CR3: 0000000fa3bb9000 CR4: 00000000000006b0
[   43.509792] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[   43.509792] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[   43.509792] Stack:
[   43.509792]  0000e5180000000e ffffffff85ca1900 ffff880fa38710d8 ffff880fa3413d98
[   43.509792]  0000000000000120 0000000000000000 ffff880fa3a98000 0000000000000000
[   43.509792]  ffffffff85cf32a0 0000000000000000 ffff880fa3413dc8 ffffffff818f3c72
[   43.509792] Call Trace:
[   43.509792]  [<ffffffff818f3c72>] ceph_mount+0xa2/0x390
[   43.509792]  [<ffffffff81226314>] ? pcpu_alloc+0x334/0x3c0
[   43.509792]  [<ffffffff81282f8d>] mount_fs+0x8d/0x1a0
[   43.509792]  [<ffffffff812263d0>] ? __alloc_percpu+0x10/0x20
[   43.509792]  [<ffffffff8129f799>] vfs_kern_mount+0x79/0x100
[   43.509792]  [<ffffffff812a224d>] do_new_mount+0xcd/0x1c0
[   43.509792]  [<ffffffff812a2e8d>] do_mount+0x15d/0x210
[   43.509792]  [<ffffffff81220e55>] ? strndup_user+0x45/0x60
[   43.509792]  [<ffffffff812a2fdd>] SyS_mount+0x9d/0xe0
[   43.509792]  [<ffffffff83fd816c>] tracesys+0xdd/0xe2
[   43.509792] Code: 4c 8b 5d c0 74 0a 48 8d 50 01 49 89 14 24 eb 17 31 c0 48 83 c9 ff $
[   43.509792] RIP  [<ffffffff818f3884>] parse_mount_options+0x1a4/0x300
[   43.509792]  RSP <ffff880fa3413d08>
[   43.509792] CR2: ffff880fa3a97fff
[   43.509792] ---[ end trace 22469cd81e93af51 ]---

Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Reviewed-by: Sage Weil <sage@inktan.com>
possible btrfs fix from chris