Quick fix: revert #3341 due to regression

[homm]

from PIL import Image, ImageOps

with open('input/256.webp', 'rb') as blob:
    Image.open(blob).load()
    Image.open(blob).load()

/home/homm/Code/Pillow/src/PIL/Image.pyc in open(fp, mode)
   2611 
   2612     try:
-> 2613         fp.seek(0)
   2614     except (AttributeError, io.UnsupportedOperation):
   2615         fp = io.BytesIO(fp.read())

ValueError: I/O operation on closed file

[homm]

@radarhere @hugovk I reverting #3341. Warning is better than ValueError.

Please, look for a better way to fix it, probably with _exclusive_fp check.

[hugovk]

@homm How does PR #3418 look?