btrfs: try harder to allocate raid56 stripe cache

The stripe hash table is large, starting with allocation order 4 and can go as
high as order 7 in case lock debugging is turned on and structure padding
happens.

Observed mount failure:

mount: page allocation failure: order:7, mode:0x200050
Pid: 8234, comm: mount Tainted: G        W    3.8.0-default+ #267
Call Trace:
 [<ffffffff81114353>] warn_alloc_failed+0xf3/0x140
 [<ffffffff811171d2>] ? __alloc_pages_direct_compact+0x92/0x250
 [<ffffffff81117ac3>] __alloc_pages_nodemask+0x733/0x9d0
 [<ffffffff81152878>] ? cache_alloc_refill+0x3f8/0x840
 [<ffffffff811528bc>] cache_alloc_refill+0x43c/0x840
 [<ffffffff811302eb>] ? is_kernel_percpu_address+0x4b/0x90
 [<ffffffffa00a00ac>] ? btrfs_alloc_stripe_hash_table+0x5c/0x130 [btrfs]
 [<ffffffff811531d7>] kmem_cache_alloc_trace+0x247/0x270
 [<ffffffffa00a00ac>] btrfs_alloc_stripe_hash_table+0x5c/0x130 [btrfs]
 [<ffffffffa003133f>] open_ctree+0xb2f/0x1f90 [btrfs]
 [<ffffffff81397289>] ? string+0x49/0xe0
 [<ffffffff813987b3>] ? vsnprintf+0x443/0x5d0
 [<ffffffffa0007cb6>] btrfs_mount+0x526/0x600 [btrfs]
 [<ffffffff8115127c>] ? cache_alloc_debugcheck_after+0x4c/0x200
 [<ffffffff81162b90>] mount_fs+0x20/0xe0
 [<ffffffff8117db26>] vfs_kern_mount+0x76/0x120
 [<ffffffff811801b6>] do_mount+0x386/0x980
 [<ffffffff8112a5cb>] ? strndup_user+0x5b/0x80
 [<ffffffff81180840>] sys_mount+0x90/0xe0
 [<ffffffff81962e99>] system_call_fastpath+0x16/0x1b

Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>