mm/mremap: refactor mremap() system call implementation

Place checks into a separate function so the mremap() system call is less
egregiously long, remove unnecessary mremap_to() offset_in_page() check
and just check that earlier so we keep all such basic checks together.

Separate out the VMA in-place expansion, hugetlb and expand/move logic
into separate, readable functions.

De-duplicate code where possible, add comments and ensure that all error
handling explicitly specifies the error at the point of it occurring
rather than setting a prefixed error value and implicitly setting (which
is bug prone).

This lays the groundwork for subsequent patches further simplifying and
extending the mremap() implementation.

Link: https://lkml.kernel.org/r/fc4a925396dc3cc36791ec92c4d329209e816308.1741639347.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: lorenzo-stoakes