block: handle invalid lseek returns gracefully

In commit 223a23c1, we implemented a
workaround in the gluster driver to handle invalid values returned for
SEEK_DATA or SEEK_HOLE.

In some instances, these same invalid values can be seen in the posix
file handler as well - for example, it has been reported on FUSE gluster
mounts.

Calling assert() for these invalid values is overly harsh; we can safely
return -EIO and allow this case to be treated as a "learned nothing"
case (e.g., D4 / H4, as commented in the code).

This patch does the same thing that 223a23c1 did for gluster.c,
except in file-posix.c
Signed-off-by: NJeff Cody <jcody@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/file-posix.c

@@ -2114,7 +2114,12 @@ static int find_allocation(BlockDriverState *bs, off_t start,
     if (offs < 0) {
         return -errno;          /* D3 or D4 */
     }
-    assert(offs >= start);
+
+    if (offs < start) {
+        /* This is not a valid return by lseek().  We are safe to just return
+         * -EIO in this case, and we'll treat it like D4. */
+        return -EIO;
+    }
 
     if (offs > start) {
         /* D2: in hole, next data at offs */
@@ -2146,7 +2151,12 @@ static int find_allocation(BlockDriverState *bs, off_t start,
     if (offs < 0) {
         return -errno;          /* D1 and (H3 or H4) */
     }
-    assert(offs >= start);
+
+    if (offs < start) {
+        /* This is not a valid return by lseek().  We are safe to just return
+         * -EIO in this case, and we'll treat it like H4. */
+        return -EIO;
+    }
 
     if (offs > start) {
         /*