fold fs_struct->{lock,seq} into a seqlock

	The combination of spinlock_t lock and seqcount_spinlock_t seq
in struct fs_struct is an open-coded seqlock_t (see linux/seqlock_types.h).
	Combine and switch to equivalent seqlock_t primitives.  AFAICS,
that does end up with the same sequence of underlying operations in all
cases.
	While we are at it, get_fs_pwd() is open-coded verbatim in
get_path_from_fd(); rather than applying conversion to it, replace with
the call of get_fs_pwd() there.  Not worth splitting the commit for that,
IMO...

	A bit of historical background - conversion of seqlock_t to
use of seqcount_spinlock_t happened several months after the same
had been done to struct fs_struct; switching fs_struct to seqlock_t
could've been done immediately after that, but it looks like nobody
had gotten around to that until now.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Link: https://lore.kernel.org/20250702053437.GC1880847@ZenIV


Acked-by: Ahmed S. Darwish <darwi@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>