ANDROID: f2fs: Place read_waiters usage under conditional check

The `read_waiters` variable in wait_queue_head_t structure is defined
conditionally depending on the CONFIG_F2FS_UNFAIR_RWSEM kernel config
option and thus all its usages must be placed under ifdef clauses.
Enclose it's usage appropriately to prevent compilation errors.

Test: Build with/without CONFIG_F2FS_UNFAIR_RWSEM option
Bug: 377445619

Change-Id: I09fcd3f3434462d4a455f7671e1aee88b3213538
Signed-off-by: Hubert Mazur <hmazur@google.com>