fs: add asserting functions for sb_start_{write,pagefault,intwrite}

fs: add asserting functions for sb_start_{write,pagefault,intwrite}

Add an assert function sb_assert_write_started() to check if
sb_start_write() is properly called. It is used in the next commit.

Also, add the assert functions for sb_start_pagefault() and
sb_start_intwrite().

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>