fs: move fd_install() slowpath into a dedicated routine and provide commentary

fs: move fd_install() slowpath into a dedicated routine and provide commentary

On stock kernel gcc 14 emits avoidable register spillage:
	endbr64
	call   ffffffff81374630 <__fentry__>
	push   %r13
	push   %r12
	push   %rbx
	sub    $0x8,%rsp
	[snip]

Total fast path is 99 bytes.

Moving the slowpath out avoids it and shortens the fast path to 74
bytes.

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://patch.msgid.link/20251110095634.1433061-1-mjguzik@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>