mm/mremap: check remap conditions earlier

When we expand or move a VMA, this requires a number of additional checks
to be performed.

Make it really obvious under what circumstances these checks must be
performed and aggregate all the checks in one place by invoking this in
check_prep_vma().

We have to adjust the checks to account for shrink + move operations by
checking new_len <= old_len rather than new_len == old_len.

No functional change intended.

[lorenzo.stoakes@oracle.com: allow undocumented mremap() shrink behaviour]
  Link: https://lkml.kernel.org/r/8fc92a38-c636-465e-9a2f-2c6ac9cb49b8@lucifer.local
Link: https://lkml.kernel.org/r/8b4161ce074901e00602a446d81f182db92b0430.1752770784.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>