LU-9679 osc: simplify osc_page_gang_lookup()

LU-9679 osc: simplify osc_page_gang_lookup()

osc_page_gang_lookup() has 4 values that it can receive from a
callback, and that it can return to the caller:
	CLP_GANG_OKAY,
	CLP_GANG_RESCHED,
	CLP_GANG_AGAIN,
	CLP_GANG_ABORT

"AGAIN" is never used.
"RESCHED" is not needed as a cond_resched() can safely be called at
the point this is returned, rather than returning it.
That leaves "OKAY" and "ABORT" which can simply by "true" and "false"
boolean values.

Internalizing the RESCHED case means the callers don't need to loop
themselves.  This simplifies calling patterns.

Linux-Commit: 1e8fd6f9806c ("lustre: osc_cache: simplify
	osc_page_gang_lookup()")

Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Signed-off-by: Mr NeilBrown <neilb@suse.com>
Change-Id: I603963b72e4299bebcdaf4064428d6281fd12def
Reviewed-on: https://review.whamcloud.com/37599
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>