io_uring/zcrx: reorder fd allocation in zcrx_export()

io_uring/zcrx: reorder fd allocation in zcrx_export()

Currently, zcrx_export() allocates a file descriptor and copies the
control structure to userspace before the backing file is created.

While the operation returns an error on failure, it is cleaner to
follow the standard kernel pattern of performing the copy_to_user()
and fd_install() only after all resource allocations (like the
anon_inode) have succeeded. This aligns the code with other
fd-publishing paths in the VFS.

Signed-off-by: Bertie Tryner <Bertie.Tryner@warwick.ac.uk>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://patch.msgid.link/1513a3f4ae7161692ca6e991b9f01278a6bc60e4.1779189667.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>