seq_file: Simplify __seq_puts()

Change the implementation of the out-of-line __seq_puts() to simply be
a seq_write() call instead of duplicating the overflow/memcpy logic.

Suggested-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/7cebc1412d8d1338a7e52cc9291d00f5368c14e4.1713781332.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Christian Brauner <brauner@kernel.org>