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(), zfs_write(), or
zfs_freesp().  Linux 3.18 removed the lock_may_read() and
lock_may_write() interfaces which we were reyling on for this
purpose.  Rather than emulating those interfaces we remove the
redundant checks.

Fixes #2804

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