erofs: use read_cache_page_gfp for erofs_get_meta_page

As Christoph said [1], "I'd much prefer to just use
read_cache_page_gfp, and live with the fact that this
allocates bufferheads behind you for now.  I'll try to
speed up my attempts to get rid of the buffer heads on
the block device mapping instead. "

This simplifies the code a lot and a minor thing is
"no REQ_META (e.g. for blktrace) on metadata at all..."

[1] https://lore.kernel.org/r/20190903153704.GA2201@infradead.org/
Reported-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Link: https://lore.kernel.org/r/20190904020912.63925-26-gaoxiang25@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>