physmem: fix qemu_ram_alloc_from_fd size calculation

qemu_ram_alloc_from_fd allocates space if file_size == 0.  If non-zero,
it uses the existing space and verifies it is large enough, but the
verification was broken when the offset parameter was introduced.  As
a result, a file smaller than offset passes the verification and causes
errors later.  Fix that, and update the error message to include offset.

Peter provides this concise reproducer:

  $ touch ramfile
  $ truncate -s 64M ramfile
  $ ./qemu-system-x86_64 -object memory-backend-file,mem-path=./ramfile,offset=128M,size=128M,id=mem1,prealloc=on
  qemu-system-x86_64: qemu_prealloc_mem: preallocating memory failed: Bad address

With the fix, the error message is:
  qemu-system-x86_64: mem1 backing store size 0x4000000 is too small for 'size' option 0x8000000 plus 'offset' option 0x8000000

Cc: [email protected]
Fixes: 4b870dc ("hostmem-file: add offset option")
Signed-off-by: Steve Sistare <[email protected]>
Reviewed-by: Peter Xu <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Fabiano Rosas <[email protected]>

Author: Steve Sistare

system/physmem.c

@@ -1970,10 +1970,12 @@ RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
     size = REAL_HOST_PAGE_ALIGN(size);
 
     file_size = get_file_size(fd);
-    if (file_size > offset && file_size < (offset + size)) {
-        error_setg(errp, "backing store size 0x%" PRIx64
-                   " does not match 'size' option 0x" RAM_ADDR_FMT,
-                   file_size, size);
+    if (file_size && file_size < offset + size) {
+        error_setg(errp, "%s backing store size 0x%" PRIx64
+                   " is too small for 'size' option 0x" RAM_ADDR_FMT
+                   " plus 'offset' option 0x%" PRIx64,
+                   memory_region_name(mr), file_size, size,
+                   (uint64_t)offset);
         return NULL;
     }