jfs: jfs_xtree: replace XT_GETPAGE macro with xt_getpage()

Replace legacy XT_GETPAGE macro with an inline function that returns a
xtpage_t pointer and update all instances of XT_GETPAGE in jfs_xtree.c

Signed-off-by: Suchit Karunakaran <suchitkarunakaran@gmail.com>

Simplified xt_getpage by removing size and rc arguments.

Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>