LU-12043 llite: move tunable params to sysfs_memparse()

LU-12043 llite: move tunable params to sysfs_memparse()

Move the max_read_ahead_* tunables from debugfs to sysfs, since
they follow the one-value-per-file rule and should be visible to
regular users.

Rename the functions and constants from *readahead* to *read_ahead*
or *READ_AHEAD* to match the tunable names from procfs.

Deprecate usage of llprocfs_str_with_units_to_s64(), lu_str_to_s64(),
llprocfs_str_with_units_to_u64(), and lu_str_to_u64(), and instead
use sysfs_memparse() to parse sizes in the few remaining places
where they are used.  A separate patch will remove those functions.

Minor fix to the "lctl set_param" man page.

Fixes: adb5aca3d673 ("LU-8066 llite: Move all remaining procfs entries to debugfs")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I2cdf5f8f0aeca458ed1989366102c33ae83ebbe5
Reviewed-on: https://review.whamcloud.com/34849
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>