btrfs: send: fix -Wflex-array-member-not-at-end warning in struct send_ctx

The warning -Wflex-array-member-not-at-end was introduced in GCC-14, and
we are getting ready to enable it, globally.

Fix the following warning:

  fs/btrfs/send.c:181:24: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

and move the declaration of send_ctx::cur_inode_path to the end.

Notice that struct fs_path contains a flexible array member inline_buf,
but also a padding array and a limit calculated for the usable space of
inline_buf (FS_PATH_INLINE_SIZE). It is not the pattern where flexible
array is in the middle of a structure and could potentially overwrite
other members.

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>