scsi: qla2xxx: Use mutex protection during qla2x00_sysfs_read_fw_dump()

Add mutex protection to prevent driver from freeing the FW dump buffer
while the extraction is in progress.

[mkp: commit desc]

Signed-off-by: Quinn Tran <quinn.tran@cavium.com>
Signed-off-by: Himanshu Madhani <hmadhani@marvell.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>