mm/filemap/c: break generic_file_buffered_read up into multiple functions

Patch series "generic_file_buffered_read() improvements", v2.

generic_file_buffered_read() has turned into a real monstrosity to work
with.  And it's a major performance improvement, for both small random and
large sequential reads.  On my test box, 4k buffered random reads go from
~150k to ~250k iops, and the improvements to big sequential reads are even
bigger.

This incorporates the fix for IOCB_WAITQ handling that Jens just posted as
well, also factors out lock_page_for_iocb() to improve handling of the
various iocb flags.

This patch (of 2):

This is prep work for changing generic_file_buffered_read() to use
find_get_pages_contig() to batch up all the pagecache lookups.

This patch should be functionally identical to the existing code and
changes as little as of the flow control as possible.  More refactoring
could be done, this patch is intended to be relatively minimal.

Link: https://lkml.kernel.org/r/20201025212949.602194-1-kent.overstreet@gmail.com
Link: https://lkml.kernel.org/r/20201025212949.602194-2-kent.overstreet@gmail.com
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>