fs: Split fcntl_rw_hint()

Split fcntl_rw_hint() such that there is one helper function per fcntl.
Use READ_ONCE() and WRITE_ONCE() to access the i_write_hint member
instead of protecting such accesses with the inode lock. READ_ONCE() is
not used in I/O path code that reads i_write_hint. Users who want
F_SET_RW_HINT to affect I/O need to make sure that F_SET_RW_HINT has
completed before I/O is submitted that should use the configured write
hint.

Cc: Christoph Hellwig <hch@lst.de>
Suggested-by: Christoph Hellwig <hch@lst.de>
Cc: Kanchan Joshi <joshi.k@samsung.com>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: Chuck Lever <chuck.lever@oracle.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20240202203926.2478590-4-bvanassche@acm.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

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