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 <[email protected]>
Suggested-by: Christoph Hellwig <[email protected]>
Cc: Kanchan Joshi <[email protected]>
Cc: Jeff Layton <[email protected]>
Cc: Chuck Lever <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Stephen Rothwell <[email protected]>
Signed-off-by: Bart Van Assche <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Christian Brauner <[email protected]>

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