mm: Disable readahead by default

An article from 2014 (old but relevant) discussed the impact of
readahead on flash storage and its effects on SQLine and DBMS
performance. What was discovered was that turning off readahead could
improve performance and power consumption by 70% (sequential; 60% for
random).

https://www.semanticscholar.org/paper/Revisiting-read-ahead-efficiency-for-raw-NAND-flash-Olivier-Boukhobza/297f3d189dcd9b099f8f6b5fbc7e304db395a683

Signed-off-by: Tyler Nijmeh <tylernij@gmail.com>

Author: tytydraco

include/linux/mm.h

@@ -2161,8 +2161,8 @@ int write_one_page(struct page *page, int wait);
 void task_dirty_inc(struct task_struct *tsk);
 
 /* readahead.c */
-#define VM_MAX_READAHEAD	128	/* kbytes */
-#define VM_MIN_READAHEAD	16	/* kbytes (includes current page) */
+#define VM_MAX_READAHEAD	0	/* kbytes */
+#define VM_MIN_READAHEAD	0	/* kbytes (includes current page) */
 
 int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
 			pgoff_t offset, unsigned long nr_to_read);