mm/mremap: introduce and use vma_remap_struct threaded state

A number of mremap() calls both pass around and modify a large number of
parameters, making the code less readable and often repeatedly having to
determine things such as VMA, size delta, and more.

Avoid this by using the common pattern of passing a state object through
the operation, updating it as we go.  We introduce the vma_remap_struct or
'VRM' for this purpose.

This also gives us the ability to accumulate further state through the
operation that would otherwise require awkward and error-prone pointer
passing.

We can also now trivially define helper functions that operate on a VRM
object.

This pattern has proven itself to be very powerful when implemented for
VMA merge, VMA unmapping and memory mapping operations, so it is
battle-tested and functional.

We both introduce the data structure and use it, introducing helper
functions as needed to make things readable, we move some state such as
mmap lock and mlock() status to the VRM, we introduce a means of
classifying the type of mremap() operation and de-duplicate the
get_unmapped_area() lookup.

We also neatly thread userfaultfd state throughout the operation.

Note that there is further refactoring to be done, chiefly adjust
move_vma() to accept a VRM parameter.  We defer this as there is
pre-requisite work required to be able to do so which we will do in a
subsequent patch.

Link: https://lkml.kernel.org/r/27951739dc83b2b1523b81fa9c009ba348388d40.1741639347.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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