mount: simplify /proc/self/mountinfo handler
    
    Our IO handler is only installed for one fd, hence there's no reason to
    conditionalize on it again.
    
    Also, split out the draining into a helper function of its own.
    
    (cherry picked from commit fcd8e119c28be19ffbc5227089cf4d3b8ba60238)
    
    Conflicts:
    	src/core/mount.c
    
    Related: #1696178
    
    patch_name: 0212-mount-simplify-proc-self-mountinfo-handler.patch
    present_in_specfile: true
    location_in_specfile: 212
    squash_commits: true