gateway: add open flag when downloading an object

[SandyXSD]

The GetObjectNInfo returns a GetObjectReader, which is used to read data, so we'd better add the 'ro' flag when opening the file.
Without it, the file reader is not properly closed, leading to memory leak in a short time (will be released after 1 min by the background check).

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 55.55%. Comparing base (4026264) to head (b55e949).
Report is 1 commits behind head on main.

Files	Patch %	Lines
cmd/object.go	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4507      +/-   ##
==========================================
- Coverage   55.58%   55.55%   -0.03%     
==========================================
  Files         157      157              
  Lines       42337    42337              
==========================================
- Hits        23531    23521      -10     
- Misses      16176    16183       +7     
- Partials     2630     2633       +3     

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