crimson: bump lru size from 64M->1G, clarify description

Retain old value for qa coverage.

Signed-off-by: Samuel Just <[email protected]>
(cherry picked from commit 1824937)

Author: athanatos

qa/config/crimson_qa_overrides.yaml

@@ -10,6 +10,7 @@ overrides:
       osd:
         crimson osd obc lru size: 10
         debug ms: 20
+        seastore cache lru size: 64M
     flavor: crimson
   workunit:
     env:

src/common/options/crimson.yaml.in

@@ -150,8 +150,8 @@ options:
 - name: seastore_cache_lru_size
   type: size
   level: advanced
-  desc: Size in bytes of extents to keep in cache.
-  default: 64_M
+  desc: Size in bytes of extents to keep in cache (per reactor).
+  default: 2_G
 - name: seastore_obj_data_write_amplification
   type: float
   level: advanced