support splice reads on seq_file based procfs files v2

[Christoph Hellwig]

Hi Al,

Greg reported a problem due to the fact that Android tests use procfs
files to test splice, which stopped working with 5.10-rc1.  This series
adds read_iter support for seq_file, and uses those for various seq_files
in procfs to restore splice read support.

Chances since v1:
 - only switch a subset of proc files to use seq_read_iter