Remove checks for mandatory locks

The Linux VFS handles mandatory locks generically so we shouldn't need
to check for conflicting locks in zfs_read() and zfs_write().  Linux
3.18 removed the lock_may_read() and lock_may_write() interfaces which
we were reyling on to perform these checks.  Rather than emulating those
interfaces we remove the redundant checks.

Issue #2804

Signed-off-by: Ned Bass <bass6@llnl.gov>