readahead: fold try_context_readahead() into its single caller

try_context_readahead() has a single caller page_cache_sync_ra().  Fold
the function there to make ra state modifications more obvious.  No
functional changes.

Link: https://lkml.kernel.org/r/20240625101909.12234-9-jack@suse.cz


Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Tested-by: Zhang Peng <zhangpengpeng0808@gmail.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>