fs/ntfs3: Fix integer overflow in ni_fiemap with fiemap_prep()

Use fiemap_prep() to check valid flags. It also shrink request scope
(@len) to what the fs can actually handle.

This address following Smatch static checker warning:
	fs/ntfs3/frecord.c:1894 ni_fiemap()
	warn: potential integer overflow from user 'vbo + len'

Because fiemap_prep() shrinks @len this cannot happened anymore.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: lore.kernel.org/ntfs3/20210825080440.GA17407@kili/

Fixes: 4342306f ("fs/ntfs3: Add file operations and implementation")
Signed-off-by: Kari Argillander <kari.argillander@gmail.com>
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>