Fix a WARN_ON failure in bond_masters sysfs file

Got a report of this warning recently

bonding: bond0 is being created...
------------[ cut here ]------------
WARNING: at fs/proc/generic.c:590 proc_register+0x14d/0x185()
Hardware name: ProLiant BL465c G1
proc_dir_entry 'bonding/bond0' already registered
Modules linked in: bonding ipv6 tg3 bnx2 shpchp amd64_edac_mod edac_core
ipmi_si
ipmi_msghandler serio_raw i2c_piix4 k8temp edac_mce_amd hpwdt microcode hpsa
cc
iss radeon ttm drm_kms_helper drm i2c_algo_bit i2c_core [last unloaded:
scsi_wai
t_scan]
Pid: 935, comm: ifup-eth Not tainted 2.6.33.5-124.fc13.x86_64 #1
Call Trace:
[<ffffffff8104b54c>] warn_slowpath_common+0x77/0x8f
[<ffffffff8104b5b1>] warn_slowpath_fmt+0x3c/0x3e
[<ffffffff8114bf0b>] proc_register+0x14d/0x185
[<ffffffff8114c20c>] proc_create_data+0x87/0xa1
[<ffffffffa0211e9b>] bond_create_proc_entry+0x55/0x95 [bonding]
[<ffffffffa0215e5d>] bond_init+0x95/0xd0 [bonding]
[<ffffffff8138cd97>] register_netdevice+0xdd/0x29e
[<ffffffffa021240b>] bond_create+0x8e/0xb8 [bonding]
[<ffffffffa021c4be>] bonding_store_bonds+0xb3/0x1c1 [bonding]
[<ffffffff812aec85>] class_attr_store+0x27/0x29
[<ffffffff8115423d>] sysfs_write_file+0x10f/0x14b
[<ffffffff81101acf>] vfs_write+0xa9/0x106
[<ffffffff81101be2>] sys_write+0x45/0x69
[<ffffffff81009b02>] system_call_fastpath+0x16/0x1b
---[ end trace a677c3f7f8b16b1e ]---
bonding: Bond creation failed.

It happens because a user space writer to bond_master can try to
register an already existing bond interface name.  Fix it by teaching
bond_create to check for the existance of devices with that name first
in cases where a non-NULL name parameter has been passed in
Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>