dm mpath: add mutex to synchronize adding and flushing work
dm mpath: add mutex to synchronize adding and flushing work

Add a mutex to allow possible creators of new work to synchronize with
flushing work queues.

Signed-off-by: Mike Anderson <e49ab6407d59bd294897d20e399f2272f7e1c708@linux.vnet.ibm.com>
Acked-by: Kiyoshi Ueda <edfa28af98962b8d178d340ad7f4ac897b3af58a@ct.jp.nec.com>
Signed-off-by: Alasdair G Kergon <620085386a2c64ec2f1d43bef997c15003f13da2@redhat.com>

TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs