LU-13212 osc: fall back to vmalloc for large RPCs

LU-13212 osc: fall back to vmalloc for large RPCs

For large RPC sizes (16MB+) the page array (4096 brw_page) can
become very large (128KB+ with fscrypt) and should fall back to
vmalloc() if kmalloc() fails due to memory fragmentation.

The mdc/mdt allocations are currently limited to 1MB for readdir
RPCs, but it doesn't hurt to prepare them for larger RPCs from
clients in the future if this limit is increased.

Fixes: 51b32ac2b9b8 ("LU-7990 rpc: increase bulk size")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I56805f5701d6850412664ce0681a1456b9405580
Reviewed-on: https://review.whamcloud.com/43281
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Wang Shilong <wshilong@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>