fs: clear or set FMODE_LSEEK based on llseek function

Pipe-like behaviour on llseek(2) (i.e. unconditionally failing with
-ESPIPE) can be expresses in 3 ways:
	1) ->llseek set to NULL in file_operations
	2) ->llseek set to no_llseek in file_operations
	3) FMODE_LSEEK *not* set in ->f_mode.

Enforce (3) in cases (1) and (2); that will allow to simplify the
checks and eventually get rid of no_llseek boilerplate.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>