fallocate: add page (#2089)

pages/linux/fallocate.md

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+# fallocate 
+
+> Reserve or deallocate disk space to files.
+> The utility allocates space without zeroing.
+
+- Reserve a file taking up 700MB of disk space:
+
+`fallocate --length {{700M}} {{path/to/file}}`
+
+- Shrink an already allocated file by 200MB:
+
+`fallocate --collapse-range --length {{200M}} {{path/to/file}}`
+
+- Shrink 20MB of space after 100MB in a file:
+
+`fallocate --collapse-range --offset {{100M}} --length {{20M}} {{path/to/file}}`