sys/param.h: Add DEV_BSIZE and make use of it.

POSIX-2024 says:

> There is no correlation between values of the st_blocks and
> st_blksize, and the f_bsize (from <sys/statvfs.h>) structure members.

Some code in Haiku has mixed that up in the past (e.g. the write_overlay).
This will allow a constant to be used instead, clarifying what's happening.

POSIX-2024 does not actually require this constant, but it does say:

> Traditionally, some implementations defined the multiplier for
> st_blocks in <sys/param.h> as the symbol DEV_BSIZE.

And indeed, glibc, musl, and FreeBSD (at least) all define it.
So it seems to make sense for us to do the same.

Related to #19251.

headers/private/fs_shell/fssh_api_wrapper.h

@@ -154,6 +154,10 @@
 #define B_MIME_TYPE_LENGTH	FSSH_B_MIME_TYPE_LENGTH
 #define B_MAX_SYMLINKS		FSSH_B_MAX_SYMLINKS
 
+// Params
+
+#define DEV_BSIZE			FSSH_DEV_BSIZE
+
 // Open Modes
 #define B_READ_ONLY			FSSH_B_READ_ONLY
 #define B_WRITE_ONLY		FSSH_B_WRITE_ONLY

headers/private/fs_shell/fssh_defs.h

@@ -24,6 +24,9 @@
 #define FSSH_B_MIME_TYPE_LENGTH		(FSSH_B_ATTR_NAME_LENGTH - 15)
 #define FSSH_B_MAX_SYMLINKS			16
 
+// Params
+#define FSSH_DEV_BSIZE				512
+
 // Open Modes
 #define FSSH_B_READ_ONLY 		FSSH_O_RDONLY	// read only
 #define FSSH_B_WRITE_ONLY 		FSSH_O_WRONLY 	// write only