LU-9417 mdc: excessive memory consumption by the xattr cache

LU-9417 mdc: excessive memory consumption by the xattr cache

The refill operation of the xattr cache does not know the
reply size in advance, so it makes a guess based on
the maxeasize value returned by the MDS.

In practice, it allocates 16 KiB for the common case and
4 MiB for the large xattr case. However, a typical reply
is just a few hundred bytes. Even worse, RHEL6 has a very
bad vmalloc()/vfree() design with global locks.

If we follow the conservative approach, we can prepare a
single memory page for the reply. It is large enough for
any reasonable xattr set and, at the same time, it does
not require multiple page memory reclaim, which can be
costly.

If, for a specific file, the reply is larger than a single
page, the client is prepared to handle that and will fall back
to non-cached xattr code. Indeed, if this happens often and
xattrs are often used to store large values, it makes sense to
disable the xattr cache at all since it wasn't designed for
such [mis]use.

Change-Id: I98d97ffea5c83bccbc9f254717af8d2c0ac4d77f
Signed-off-by: Andrew Perepechko <andrew.perepechko@seagate.com>
Reviewed-on: https://review.whamcloud.com/26887
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Ben Evans <bevans@cray.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>