Linux compat 4.18: check_disk_size_change()

Added support for the bops->check_events() interface which was
added in the 2.6.38 kernel to replace bops->media_changed().
Fully implementing this functionality allows the volume resize
code to rely on revalidate_disk(), which is the preferred
mechanism, and removes the need to use check_disk_size_change().

In order for bops->check_events() to lookup the zvol_state_t
stored in the disk->private_data the zvol_state_lock needs to
be held.  Since the check events interface may poll the mutex
has been converted to a rwlock for better concurrently.  The
rwlock need only be taken as a writer in the zvol_free() path
when disk->private_data is set to NULL.

The configure checks for the block_device_operations structure
were consolidated in a single kernel-block-device-operations.m4
file.

The ZFS_AC_KERNEL_BDEV_BLOCK_DEVICE_OPERATIONS configure checks
and assoicated dead code was removed.  This interface was added
to the 2.6.28 kernel which predates the oldest supported 2.6.32
kernel and will therefore always be available.

Updated maximum Linux version in META file.  The 4.17 kernel
was released on 2018-06-03 and ZoL is compatible with the
finalized kernel.

Reviewed-by: Boris Protopopov <[email protected]>
Reviewed-by: Sara Hartse <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes #7611 

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