LU-12455 osd: Correct readcache_max_filesize proc

LU-12455 osd: Correct readcache_max_filesize proc

Readcache max file size is displayed as unsigned, but
parsed as signed.  This causes confusion when users try to
set it back to the default, which is 2^64-1 (or -1), and
they can't use 2^64-1.

Easy enough to add an unsigned parser and use it instead.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I15f5677c61f4f12a8448a665c5d52a8c94d062f3
Reviewed-on: https://review.whamcloud.com/35266
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Ann Koehler <amk@cray.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>