From: Bob Peterson
Starting with patch 2cb1e08985, gfs2 started using the new function
filemap_splice_read rather than the old (and subsequently deleted)
function generic_file_splice_read.
filemap_splice_read works by taking references to a number of folios in
the page cache and splicing those
Function gfs2_trans_add_meta() checks for the SDF_FROZEN flag to make
sure that no buffers are added to a transaction while the filesystem is
frozen. With the recent freeze/thaw rework, the SDF_FROZEN flag is
cleared after thaw_super() is called, which is sufficient for
serializing freeze/thaw.
