fs: sort out the fallocate mode vs flag mess

The fallocate system call takes a mode argument, but that argument
contains a wild mix of exclusive modes and an optional flags.

Replace FALLOC_FL_SUPPORTED_MASK with FALLOC_FL_MODE_MASK, which excludes
the optional flag bit, so that we can use switch statement on the value
to easily enumerate the cases while getting the check for duplicate modes
for free.

To make this (and in the future the file system implementations) more
readable also add a symbolic name for the 0 mode used to allocate blocks.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20240827065123.1762168-4-hch@lst.de
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

include/uapi/linux/falloc.h

@@ -2,6 +2,7 @@
 #ifndef _UAPI_FALLOC_H_
 #define _UAPI_FALLOC_H_
 
+#define FALLOC_FL_ALLOCATE_RANGE 0x00 /* allocate range */
 #define FALLOC_FL_KEEP_SIZE	0x01 /* default is extend size */
 #define FALLOC_FL_PUNCH_HOLE	0x02 /* de-allocates range */
 #define FALLOC_FL_NO_HIDE_STALE	0x04 /* reserved codepoint */