Some more minor cleanups of em(4) driver:

• Destroy mutex in case of attach failure. [1]
• Lock properly em_watchdog(). [1]
• Lock properly em_sysctl_int_delay(). [1]
• Remove unused global adapter linked list.
• Remove unused dma_size field from struct em_dma_alloc.
• Do not touch interface statistics, that must be edited only by upper layers. [1]

Submitted by: yongari [1]