ramfs: fix nommu mmap with gaps in the page cache

ramfs: fix nommu mmap with gaps in the page cache

ramfs needs to check that pages are both physically contiguous and
contiguous in the file.  If the page cache happens to have, eg, page A for
index 0 of the file, no page for index 1, and page A+1 for index 2, then
an mmap of the first two pages of the file will succeed when it should
fail.

Fixes: 642fb4d1f1dd ("[PATCH] NOMMU: Provide shared-writable mmap support on ramfs")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: David Howells <dhowells@redhat.com>
Link: https://lkml.kernel.org/r/20200914122239.GO6583@casper.infradead.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>