system/physmem: take into account fd_offset for file fallocate

Punching a hole in a file with fallocate needs to take into account the
fd_offset value for a correct file location.
But guest_memfd internal use doesn't currently consider fd_offset.

Fixes: 4b870dc4d0c0 ("hostmem-file: add offset option")

Signed-off-by: William Roche <william.roche@oracle.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20250122194053.3103617-2-william.roche@oracle.com
Signed-off-by: Peter Xu <peterx@redhat.com>

system/physmem.c

@@ -3797,18 +3797,19 @@ int ram_block_discard_range(RAMBlock *rb, uint64_t start, size_t length)
             }
 
             ret = fallocate(rb->fd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
-                            start, length);
+                            start + rb->fd_offset, length);
             if (ret) {
                 ret = -errno;
-                error_report("%s: Failed to fallocate %s:%" PRIx64 " +%zx (%d)",
-                             __func__, rb->idstr, start, length, ret);
+                error_report("%s: Failed to fallocate %s:%" PRIx64 "+%" PRIx64
+                             " +%zx (%d)", __func__, rb->idstr, start,
+                             rb->fd_offset, length, ret);
                 goto err;
             }
 #else
             ret = -ENOSYS;
             error_report("%s: fallocate not available/file"
-                         "%s:%" PRIx64 " +%zx (%d)",
-                         __func__, rb->idstr, start, length, ret);
+                         "%s:%" PRIx64 "+%" PRIx64 " +%zx (%d)", __func__,
+                         rb->idstr, start, rb->fd_offset, length, ret);
             goto err;
 #endif
         }
@@ -3855,6 +3856,7 @@ int ram_block_discard_guest_memfd_range(RAMBlock *rb, uint64_t start,
     int ret = -1;
 
 #ifdef CONFIG_FALLOCATE_PUNCH_HOLE
+    /* ignore fd_offset with guest_memfd */
     ret = fallocate(rb->guest_memfd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
                     start, length);