fs: add <linux/init_task.h> for 'init_fs'

fs: add <linux/init_task.h> for 'init_fs'

The init_fs symbol is defined in <linux/init_task.h> but was
not included in fs/fs_struct.c so fix by adding the include.

Fixes the following sparse warning:
fs/fs_struct.c:150:18: warning: symbol 'init_fs' was not declared. Should it be static?

Fixes: 3e93cd671813e ("Take fs_struct handling to new file")
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Link: https://patch.msgid.link/20260108115856.238027-1-ben.dooks@codethink.co.uk
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>