UBIFS: add no_chk_data_crc mount option

UBIFS read performance can be improved by skipping the CRC
check when data nodes are read.  This option can be used if
the underlying media is considered to be highly reliable.
Note that CRCs are always checked for metadata.

Read speed on Arm platform with OneNAND goes from 19 MiB/s
to 27 MiB/s with data CRC checking disabled.

Signed-off-by: Adrian Hunter <ext-adrian.hunter@nokia.com>

Documentation/filesystems/ubifs.txt

@@ -89,6 +89,12 @@ fast_unmount		do not commit on unmount; this option makes
 bulk_read		read more in one go to take advantage of flash
 			media that read faster sequentially
 no_bulk_read (*)	do not bulk-read
+no_chk_data_crc		skip checking of CRCs on data nodes in order to
+			improve read performance. Use this option only
+			if the flash media is highly reliable. The effect
+			of this option is that corruption of the contents
+			of a file can go unnoticed.
+chk_data_crc (*)	do not skip checking CRCs on data nodes
 
 
 Quick usage instructions