mm/mremap: put VMA check and prep logic into helper function

mm/mremap: put VMA check and prep logic into helper function

Rather than lumping everything together in do_mremap(), add a new helper
function, check_prep_vma(), to do the work relating to each VMA.

This further lays groundwork for subsequent patches which will allow for
batched VMA mremap().

Additionally, if we set vrm->new_addr == vrm->addr when prepping the VMA,
this avoids us needing to do so in the expand VMA mlocked case.

No functional change intended.

Link: https://lkml.kernel.org/r/15efa3c57935f7f8894094b94c1803c2f322c511.1752770784.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Jann Horn <jannh@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Rik van Riel <riel@surriel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>