iomap: Fix conflicting values of iomap flags

iomap: Fix conflicting values of iomap flags

IOMAP_F_ATOMIC_BIO mistakenly took the same value as of IOMAP_F_SIZE_CHANGED
in patch '370a6de7651b ("iomap: rework IOMAP atomic flags")'.
Let's fix this and let's also create some more space for filesystem reported
flags to avoid this in future. This patch makes the core iomap flags to start
from bit 15, moving downwards. Note that "flags" member within struct iomap
is of type u16.

Fixes: 370a6de7651b ("iomap: rework IOMAP atomic flags")
Signed-off-by: "Ritesh Harjani (IBM)" <ritesh.list@gmail.com>
Link: https://lore.kernel.org/r/20250327170119.61045-1-ritesh.list@gmail.com
Reviewed-by: John Garry <john.g.garry@oracle.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>