gfs2: Remove use of error flag in journal reads

Conventionally, we use the uptodate bit to signal whether a read
encountered an error or not.  Use folio_end_read() to set the uptodate
bit on success.  Also use filemap_set_wb_err() to communicate the errno
instead of the more heavy-weight mapping_set_error().

Signed-off-by: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>