Support mandatory locks (nbmand)

Support mandatory locks (nbmand)

The Linux kernel already has support for mandatory locking.  This
change just replaces the Solaris mandatory locking calls with the
Linux equivilants.  In fact, it looks like this code could be
removed entirely because this checking is already done generically
in the Linux VFS.  However, for now we'll leave it in place even
if it is redundant just in case we missed something.

The original patch to update the code to support mandatory locking
was done by Rohan Puri.  This patch is an updated version which is
compatible with the previous mount option handling changes.

Original-Patch-by: Rohan Puri <rohan.puri15@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #222
Closes #253