ixgbevf: add spinlocks for MTU change calls

ixgbevf: add spinlocks for MTU change calls

Orabug: 24568240

Protect set_rlpml with mailbox lock to make sure the MTU configuration
is handled properly.

This change resolves an issue where set_rlpml can fail when the VF
interface is brought up:
ixgbevf 0000:03:1d.6: Failed to set MTU at 1500

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
(cherry picked from commit 14b22cd9827ad6765a00ca0b267c3cb0353d9c10)
Signed-off-by: Brian Maly <brian.maly@oracle.com>